Even then it only appears so to those commenters, any thread talking about healthcare, legal rights etc etc has the same festering underbelly of hatred you'll find on any comment section from a newspaper.
The major reason you can have this perspective is that a lot of these contentious subjects are blanket banned, the inevitable firestorm in the comments can't happen if the thread isn't allowed.
The commentariat's genteel reputation also evaporates once you step into the "political" threads HN does allow, namely immigration, outsourcing and of course, crime in San Francisco.
A former head of state in Ycombinator's home country is headed to trial, and a coup plotter in the world's current largest military conflict is apparently dead. Both of these are historic events, but you'd never know that on HN.