@Kazapsky
Just because a particular example of a reasoning fallacy is widely accepted doesn’t make it any less of a fallacy. Also involves the fallacy of incomplete evidence and confirmation bias - for this kind of thing it is common for people to focus on specific negatives and ignore the body of contradicting evidence. For instance, if a person initially meets a small number of white people who are racist, and hastily generalises that all white people are racist, then in future interactions with white people they will exhibit a confirmation bias because they already have a conclusion about white people and they will tend to remember and focus on evidence that supports their pre-existing conclusion while ignoring the rest, forming their conclusion based on cherrypicking data and incomplete evidence.
It works with fandoms, too. You hear about some people who are fans of a thing doing awful stuff (because let’s face it, you’re much more likely to hear about people being awful than you are about people being good because of reporting bias), you generalise that a fandom is terrible, and from then on whenever you see evidence that a fan is not a raving lunatic you ignore it and whenever you see one that is you go “see! I told you, they’re all assholes!”