I don't understand the fine points of copyright law, but my understanding is that if he's taken unmodified art directly from the game, created by staff artists, and redistributed them, Bethesda is obligated (by contract) to have those images removed. If that's the case (I don't know the details) they're not being mean, they're fulfilling their legal obligations. It's the same reason why you can't just unpack and redistribute their textures or models.
Mod sites, which monetize their downloads (ads, subscriptions) avoid breaking copyright by not hosting original art from the game. They can monetize a site that has modded content, but not original content. Most fan sites aren't redistributing original art assets; they're creating derivative works which are easier to protect under fair use. Even in the case of a close call, most of these sites aren't worth going after.
But again, I'm no expert. That's just my understanding of it.
Bethesda seems to be trying quite hard to be jerks in an industry that is already drowning in them. It is starting to look like highschool with each game company seeing who can be the bigger bully toward the fans.