HN is a public outlet, anyone is free to register and participate in the discussion. The thread is soliciting the community to censor themselves with a pretty vague and large blanket under "politics".
HN is run by an American corporation correct, it's servers are in America, then it should follow American values. If this was a Chinese or Russian company, well, their land their rules.
People are freaking out because dang set a precedent for future discourse, the community leadership can declare any topic they deem unsuitable with a pretty vague set of rules and discretion which does not follow a democratic process.
It's true that HN doesn't follow a democratic process that way, but it never has, nor pretended to.