Because it's a special interest they're (special interestes) weird as hell.
I'm 100% sure you have a weird special interest even if you don't know it yet.
Well, Rule 34 on ponies in my view aren't right in my view and i don't think it's going to change.
Considering how new the show is I doubt people were thinking "wowe, i want to forget that underage generation-four horse!!!!!" back before it was broadcasted, it's a new special interest.