I suspect that is a misinterpretation of moderating practices here. As long as you aren't doing things like engaging in personal attacks, you are allowed to express yourself pretty freely here. However, they do penalize articles on certain subjects, in part because the discussion of some subjects typically does not go well.
It is sort of like "Yelling FIRE! in a crowded theater is not a protected form of free speech." Putting inflammatory topics with inflammatory titles etc on HN will get it downmodded (or they will change the title to something less inflammatory, etc). That doesn't mean you can't say it. It does mean they want the front page to be dominated by intelligent, meaty discussion, not ranty, fighty, strongly emotional drivel.
When things get downmodded in order to get less attention by removing them from the front page, inevitably, someone says something about how their right to free speech is being impeded or "Why can't we talk about this here?" I have seen the moderator reply to the effect of "You are talking about it still, it just isn't on the front page anymore."
Some people think "right to free speech = right to be on the front page if I get enough upvotes, no matter how much I have violated the guidelines" and will say something nasty about how things are moderated if their desire to be attention whores has been interfered with by the folks who, you know, get paid to run the place as they see fit.