Comedy is subversive, but the venue is usually not. When a comedian gives a routine on stage, the audience is primed and has certain expectations. When the audience's expectations are not aligned, it is bad for both the comedian and the audience.
My limited experiences on HN as a junior user have yielded a certain kind of fuzziness as to what is acceptable. On any given day, there is a lot of variance in the content, but usually HNish and that's a good thing. Officially, the guidelines are posted and they have an emphasis on intellectual stuff/tone, but leave evocative content unexplained. So the community decides, which is usually sporadic, hap-hazard, heavily-dependent on who posted the submission (people have expectations about what Zed Shaw is going to say), and the touchiness of the content. This probably doesn't happen in other communities that are more clearly guided as either relaxed or strict.