But, just to be clear...
I don't think anyone was labeling them Bernie supporters because they were acting violently. They were being labeled Bernie supporters because they were, you know, chanting Bernie's name. I feel like that's an important distinction to make.
But, I get your point about using groups as a weapon, but I'm not entirely sure that's what people do, although it may appear that way. When we're talking about groups of voters, it's super easy to fall into that trap. Like, if we say "As a group, gays vote for Hillary" we're not pretending that Bernie has no support among the LGBT community. If we say "Young people vote for Bernie" we're not saying Hillary gets no young votes either. I think it's just a hard and fast rule to talk about demographics when it's convenient. I don't think anyone is trying to weaponize groups against other candidates....except that minorities of all flavors are going to bitch slap Trump back to whatever orange muck he crawled out of.