I think "hellbanning" should also be a part of this conversation. The combination of mods non-transparent editing practices and hellbanning makes an extremely strong case for categorizing the mod system as a form of censorship.
HN is a private website. So, I fully believe that HN should be allowed to censor if it would like to. I certainly at times censor my friends when they post to my facebook wall. But my facebook wall isn't a community. And it doesn't purport to provide news. Thus, I believe HN should be held to a higher standard.