That's literally how legal takedown requests work. That's the norm across most websites. Letting artists treat a community content website like their own personal deviantart profiles is the exception.
There are enough problems with bad faith actors fucking with the site that adding a little bit of red tape to formalize the process is a good decision. Think of how YouTube's copyright claim system works, and how that's been abused.
The DMCA is not meant to be the first line of artist control. It was only added on to the end of the list as rule 13 on ponybooru because bad faith actors were throwing a shitfit and trying to screw with the site.
I don't think that there is anything unreasonable about what's going on there. "If you don't like the specific website rules, then here are the legal rights you have to exert control over your own copyrighted works". These rules don't even have to be described on the website itself.
These rules are here to prevent abuse by possible bad faith actors on any side, and to preserve the primary function of the site as an archive wherever possible.
If you want to talk about how what they're doing is unreasonable, then can you first try seeing things from their point of view? Both in terms of their primary operating principals as an image archive community and in terms of the many bad faith actors trying to sabotage and screw with them.