Yes, a lot of good people were fooled by Stalin at the time. However, most of them, like Pete Seeger, told the truth when they realised what he was.
I fear we may have to wait a good bit longer for the Pinochet apologists to appreciate that the guy did have his flaws...
There seems to be someting about the word "communist", which puts you beyond redemption. I suspect Pete Seeger has enough sense of humour not to be bothered too much by this.