People just don't understand. Because society prohibits exhibitions of hatred, those are most likely to seek out places that do allow it. The desired "freedom" aspect of society-free judgement is why you encounter them more often in these type of forums.
And when you got there to "repress" them, it's seen as the claws of society invading one of the few avenues that allow venting their frustrations.
Think of it like this, everywhere you go there's rules, rules, rules! You're stuck doing a job you don't like to pay bills that keep growing, having to play nice with people you barely get along with 5 to 7 days a week and only an online forum to let you vent off your stress. But then some bozo shows up and yells "no, you can't do that!".
Wouldn't that just make you angry?
This isn't about the correct thing to vent about, it's about being allowed to vent. When you tell someone they're not allowed to do something because it's bad, it usually comes off as taking away their sense of agency, and that just makes them more hateful.