What I don't get is why there is scorn in the first place?
Part of it is self-hating geek backlash (see also: sci-fi vs. fantasy nerds, Star Wars vs Star Trek, etc.). For whatever reason, a lot of geeks love to eat their own and are fiercely tribal.
I think another part of it is that that brony-hate is as much a meme as bronies themselves (see also: furries and "Amir0x is a fag" comments on YouTube videos).