@Jakku_NoShi: I mean, that, paheal, danbooru, etc. all are boorus or image boards and operate in more or less similar fashion. (Not because of same software, but because of crowdsourcing and indexing nature)
e621 does allow and even encourage artists to post (hence why we don't have such straighforward message anywhere), but the point is that when you do post your own stuff, rules and guidelines can hit you in the face much more than they would anyone else when they aren't aligned.
Additionally staff can't make exceptions every single time it's artist posting or waste hours on responding to passive aggressive blips and dmails about how they somehow have personal vendetta againts artist and how artist will now full on quit because of them.

I think derpibooru doesn't have quality standards as they are all about that friendship and allowed all and every content, but after some time I think they did enforce some rules of stuff needing to be pony related in some fashion now.