> Is it really the case?

Pretty hard to answer that question in HN with anything other than another anecdote, but I would personally answer that question with "yes". I have seen this behavior on several online forums I am in (including HN) as well as in real-world settings.

The well-documented incidents with "non-left" speakers on college campuses is another data point. Take a look at the insanity that happened at Evergreen College, for example.

This sort of behavior is easily observed, from my point of view, in real-life and online.