Most image hosting TOS are really restrictive, probably because it would be a lot more hassle to sort out just the stuff that might cause them actual legal trouble for distributing than to just blanket ban everything until you get to a super simple criterion. Like all the stuff deviantArt asks for such as proper model release forms and such if you show other people, and they only allow artistic nudity.

And of course as soon as a service is known as permissive, whether explicitly or for not enforcing their own rules, it gets used for porn a lot and then the advertisers may balk. I mean, I read that Tumblr has a porn problem, not because fandom, but because of general porn reposting that people don't have the rights to. But they by now they can't just ban every explicit pictures, because they built a user base that expects a permissive environment.