You don't need to do this for singleplayer since you can play without internet if you wanted.
I'm not a big gamer, but the last one I bought on DVD!! (Mad Max) do not starts without Steam online, so I'm not convinced, as I don't know what will be the trend at the time of BL's final relese.
But singleplayer skins are absurd. I don't know why you would even think taleworlds would even CONSIDER doing it since singleplayer native would be able to be modified at will.
No One said that those paid skins will be stored in the same format as SP models. I anticipate serious resrictions in that field.