For the most part they just don't feel that anime stuff adds any inherent value to a post, so they don't like it.
They are entitled to that opinion and that's fine. Do I think they should through a piss fit about it? No, not unless someone's using anime as an excuse to blatantly shitpost, which happens a lot to be honest. Even then they should just report the post and move on.
I'm not saying that it shouldn't make them angry, but that they should control themselves and be the change they want to see.