>>2376923/y/ does have some tolerance for borderline topics like the one's you mentioned. but a 'monster' thread has more room for variation than this one. A monster thread can contain borderline anthro, but can also contain what's basically humans in cosplay. It also helps their cause that they never have more than one thread going on. The OP of this thread not only made a thread dedicated to a single specific character (already a no-no) but it happens that that character is more anthro than monster, AND there was already another thread that they could have posted in.
As for why robot is ok, it's kind of a different topic. People are uncomfortable with furry because it has ties to bestiality, which is illegal in most places, and makes people more uncomfortable than technophilia I guess. shrug.
The rules are the rules. /y/ doesn't have many mods, so it's kind of just up to the anon's to keep things orderly and to prevent spam. Some anon's can be dicks about the rules, but after being here for as long as I have, it really is for the best. It keeps the board clean and efficient, and makes sure that there's as much variety as possible with our limited number of pages.