They're separate from this forum's mods and take a more strict approach to moderation in general. The vitriol on the Steam forums for many, many games is much worse than what can be found on those games' respective sites. I have yet to receive a death threat here (though a couple people wished for my untimely end) but one of the Steam forum mods received several since EA launch. I don't blame them for their policies, even if I'd never accept such policies here.
Personally, I think it's because everyone playing already has a Steam account, so when something happens they can go straight to the Steam board and post while those emotions are still hot, even while playing the game via the in-game overlay. A couple extra clicks makes a big difference for some people.