Not sure what you mean by highly censored, I've seen plenty of unpopular opinions on HN and people that were expressing them were never banned, in fact those users were fairly rude without being personal. Not allowing personal attacks on HN is not just a common courtesy, it protects it users from defamation, falsehoods about their life and kidnapping of topics; those are all reasons that are perfectly compatible with freedom of speech. Detaching a tread is not censoring, I never seen it happen because of uncomfortable opinions but when it becomes hugely off topic it's clearly a valid thing to do.