That's an interesting point - could you expound on it? I have a few theories myself, and I'm curious what you think the reason is.
This is why we're all supposed to be all nuanced and shit when it comes to Communist atrocities, but when it's time for the Fascist atrocities everyone has to stand up and yell HITLER HITLER HITLER. Precisely like a five-year-old.
It's just a sort of national lookin' after #1. We naturally ignore the sins closest to us and focus on those of others. Left is "self" and right is "other." Otherwise, we would need to express genuine collective guilt rather than dime-a-dozen collective contempt.
The supposed distinction between the "moderate" and "extremist" left is wildly overblown. There's no social exclusion, etc, in either direction. Nobody cares or is surprised about President Obama's association with Bill Ayers. Or, for that matter, Thoreau's with John Brown. Again, there's really nothing new here.
(I do think it's appropriate that in the nation of John Brown, we all need to walk through metal detectors to get on an airplane. Nothing could be more American than terrorism.)