Localism, or some generalization of that; it also feels like there's a kind of "principle of least privilege" here. Moderation should happen at the most local level where it's practical, with a community enforcing its own rules according to its own culture. Once you're offering a platform for people to build distinct communities on, you're assuming the position of an infrastructure provider or a government, and you have some of the obligations that go along with that.
Allowing posts on a single board doesn't carry that obligation, because that's not what you're offering; the individual posts aren't and aren't meant to be communities with their own culture. I would still say that moderators have a moral obligation to be open, accountable, and representative of the community they moderate (even if it's a community they didn't want or ask for), and HN does a particularly poor job in this regard (lobste.rs demonstrates that it's possible to run a site like this with an open moderator log).
Allowing posts on a single board doesn't carry that obligation, because that's not what you're offering; the individual posts aren't and aren't meant to be communities with their own culture. I would still say that moderators have a moral obligation to be open, accountable, and representative of the community they moderate (even if it's a community they didn't want or ask for), and HN does a particularly poor job in this regard (lobste.rs demonstrates that it's possible to run a site like this with an open moderator log).