Fred Pohl was a communist when he was a teenager, but he quit when Stalin signed a non-aggression pact with Hitler. I think, having gone to lots of sci-fi cons in the Chicago area, that fandom skews either to the far left or right.
Gene Wolf is very conservative and I always enjoy talking to him whenever I see him at a con.
Lots of people, including the earliest Christians, have been communists. The Anti-American Left, including the New York Times, never really criticised Stalin, and refused to believe blindingly obvious evidence.
And it repeats itself with Saddam.