Seems like a terrible idea.
I'll start by saying that this debate, as with most internet discussions, has been framed poorly.
Yes, I'm sure that TW has every right to do whatever it wants with the servers, including blacklisting servers that don't comply with whatever arbitrary demands it makes of them. These could be the server owner preferring cereal over croissants in the morning. Actually, I suspect that such a policy would be more legitimate than the current. I'm sure that they've had some boilerplate chucked in by the cheapest lawyer that they could find that says that they can modify their EULA and that they can take action against anyone who engages in 'unauthorised use' of their assets (which probably includes splitting them through a donation tier model).
However, having a property right doesn't make all of one's actions with that property right valid or justified. I have a right to my own sexual health and pleasure, but running around having unprotected buttsex with gay men in Earls Court in London is a pretty damn risky strategy whose risks outweigh its benefits. Likewise, banning what TW calls 'monetisation' (no doubt because of the negative connotations of the word) is a silly policy that is destructive at best or an unnecessary risk at worst.
How is it silly? I'll explain under the following headers:
Arbitrary distinctions
Server managers are allowed to monetise the server itself, but not its assets. The reasoning, I'm guessing, is that the latter is a pay-wall for 'content already paid for by the players'. This is a fairly terrible excuse:
1) The script that walls off certain items is not something the player entitled themselves to by buying the core, unmodded game. The same goes for weapon buffs and misc perks, all of which have been coded into the game by someone other than TW. It's hard to see how the novice 'buys' themselves an entitlement to these perks.
2) With regard to skins, the creation of special skins (which is very common on NW) is something done by the manager or the donating player. These skins may use existing assets, but their recombination makes them unique. This 'special skin' is not something a regular paying Warband player can access when they buy the vanilla game.
3) In the current state of the game, which is deeply monopolistic, making certain servers 'private-only' effectively excludes a player from being able to play a certain type of gamemode entirely. For instance, in NW, Minisiege has a near-monopoly of Siege activity, Tropical Paradise and Hell have a monopoly on Deathmatch, and so on. PW (the mod) is dominated by two servers. Allowing the off some of the most mature and well-administered servers to non-paying players (which is permitted under the policy) seems to defeat the expectation that the buyer will gain access an active community.
At this point, one may say that no one seriously expects a popular server to put up a paywall. But a rule that is enacted yet never expected to be enforced is a terrible rule.
Do note that, if a popular server does go private and lose its populations, the replacement public servers will likely be worse. They won't have the same scripts, the same (experienced) admins, the same maps, the same rules. They'll have to build up all of these from scratch.
4) Players are routinely cut off from items for gameplay purposes. I've only ever been able to buy all the top tier armour in a Siege match a handful of times in my life. Why? Because this initial exclusion is a gameplay mechanic. There's nothing inherently wrong with walling off skins or special weapons to paying customers, as long as you do so reasonably and limit the sort of benefits you give to donators. This makes me think that part of the reasoning behind the ban isn't monetisation per se, but rather TW trying to barge in on how, substantively, the game should be played.
In which case I wonder why TW is suddenly so keen about this very specific aspect of gameplay fairness, when it has been letting its server scripters play around with class attributes and custom weapons for ages. The inconsistency is astonishing. I can do whatever I want with the class attributes on my server, but, the second I do this for cash (as opposed to, say, wanting to privilege my veteran friends or wanting to f*ck with newbies), TW gives me the red card.
This policy is probably a way to set the ground for an items/weapons store in Bannerlord.
Pointless upstream pressure
Server hosts play no role in the day-to-day running of a server. As noted already in this thread, a host running a three figure number of servers doesn't know of, or care about, what these servers are doing. They don't have the, interest, or time to regulate these servers, not least because a 'money for skins' policy can be implemented or withdrawn by those running the server at will. So you're punishing with the threat of an IP blacklist for something that they can't control.
The issues with enforcement
While TW has taken quite a few steps to make sure that communication channels are open, I suspect that you'll have enforcement issues.
1) There has been, time-wise, a wide spread of responses to this very thread, and comparatively few people posting in it (I counted less than 20 individual posters, a minority of which are actual server managers). This makes me fear that the owners of smaller servers, who participate less in the wider community, will remain ignorant of the new policy. This will, when TW comes knocking on their door, give them less time than the managers of larger servers to adjust to a change in policy (probably a week or two vs a month and a half for those who became aware of this thread the day it was posted).
2) The policy disadvantages large servers. Highly visible servers with diffuse communities will be the first to be reported (or even monitored) by one of their players. They'll have to get rid of exclusive-access items quickly. Smaller servers with tight knit communities will be able to do more or less whatever they like. They don't attract attention by being at the top of the leaderboard. They don't need to advertise a donation system in-game - most of the players know of its existence anyway. Their website may advertise all sorts of elaborate schemes, and yet will not be found in the first place unless someone high up in TW community management has been involved with it in the past (unlikely given how out of touch you ****heads seem to be).
3) There are a billion ways to continue a donation scheme without it being advertised explicitly. Don't forget that we're a late-stage community. The number of old players who know who to contact and how the system works vastly exceeds the number of new players who need to advertised to. A perk/exclusivity system doesn't need to be open, with fancy website pages and scripts. It can run in the background and yet still confer the right amount of extra benefits to the right people.
(3.1) I can think of at least four ways in which system can be circumvented. Removing donation advertisements while keeping the skin/weapon/perk scripts is one such method.
(3.2) None of these circumventions can be combatted. Server logs (and scripts) can be faked (not that server managers would be willing to hand them over in the first place). Entrapment (asking a member of the admin team on how you can donate in exchange for walled-off skins) is insanely time-intensive and will likely result in certain key entrapment accounts being shared between members of the community or banned from the servers. Do note that this policy needs to be sustainable in the long run - any active enforcement needs to be viable over the space of five or six years (or for however long the server teams continue to make money through donations).
This is not to mention the fact that a passive enforcement policy relies on the players being aware of the ban on selling scripts in the first place. Chances are that the majority of the public community won't be.
Mini summary: passive reporting is inconsistent, active monitoring is time consuming and expensive
(3.3) A server that violates the no donation policy can easily 'lie quietly' for a while, and then continue a donation scheme through another approach. There's more than one.
(3.4) Your 'we have to right to take action against any server that goes against the spirit etc" point is redundant - you won't have a clue about any of these bypass methods in the first place unless you get lucky. There won't be a server for you to suspect in the first place, because, on the surface, everything will seem to be going as normal.
The policy serves no purpose other than petty vindication
TW does not have an alternative to the current system of discretionary donations. This is not a zero sum game. The money that would have otherwise been kept by the players is being spent on the community (mostly server hosts and scripters). Whatever is left goes to the server owners who put in the hours into making their server a platform worth donating to.
It has been said that this policy is a 'practice run' for Bannerlord. However, there's no indication that Bannerlord will have, say, a skin or weapon market for players. Even if it did, then a) well done TW, you **** up, donations are the least of your problem, and b) this would add a new, powerful justification to the policy that is currently lacking.
However, no matter which way you spin it, there's no items market on Warband, and TW seems to be more keen on dragging down successful servers for the sake of 'muh property rights' than because of lost revenue.
No, paid-for-assets are not interfering with modding
Currently, the pay-for-skins system incentives the community to create *new* combinations of skins and new scripts to implement them. How? By giving an incentive for scripters to code them, knowing that they will likely be paid for their work by script owners, who in turn know that they can likely sell these exclusive items.
Modders want to create something entirely new. I see no link between mods (which are always delivered for free) and the donations policy of a bunch of servers that the modder isn't planning to compete with. I'd be very happy to see someone try to develop upon this point further.
Miscellaneous
A couple of the the comments in this thread are clearly vindictive ("hur hur f*ck the owners"), so I'll assume that there's no actual argument behind them.
It's notable that there has been no strong 'for' argument by the developers themselves. Their justification runs to about three sentences. No dev has posted here in nearly a month. The two most senior people on this forum (Callum and Duh) are only willing to refer us to an email address, instead of addressing any of the criticism (or at least letting us know that it has been forwarded on to the devs). This seems to be the sign of a company that's hiding behind its finger. Which is unsurprising, given that TW has always been terrible at community management.
Don't forget the animosity that this will inevitably bring as you turn your community into an 'us vs TW' affair. TW didn't gain anything out of this policy. The only thing it has created is a more toxic and distrustful community, with the inevitable preemptive bans, mutual reporting by rival servers, and all the other cancer that will likely ensue.
Summary
TW has the right to do this.
The normative arguments on whether TW ought to be able to ban monetisation aren't conclusive either way.
The policy itself will be enforced inconsistently, and possibly against the wrong people (hosts instead of managers).