I've never roleplayed as a Warrior Cat or read the books. It just never really took my fancy. But I don't say stuff like "Urgh, those damn warriors" when I see cats roleplaying or see some run by. To be honest, if they don't bother or annoy me, then I have no reason to behave negatively towards them. Since I've never really affiliated with warriors, I don't really understand their 'lore', like their vocabularly or their relationships with other animals. As far as I know, they're doing what warriors do. And that's fine, as long as it doesn't disrupt my own activities.
But I think it's the way that the cats sometimes behave that bugs people a bit. Apparently, warrior cats and foxes aren't axactly best friends, and when a fox shows up somewhere, the cats go a bit crazy, even if the fox isn't involved in their roleplay. That is a bit over the top.