This of course has echoes with our own attitudes, we're far more likely to welcome a former leftie activist (one or other of the flavours of Revolutionary Communism for example) into UKIP than we are a former BNP one.
This is a matter of psychology, not philosophy. In most Western countries, people look upon Communists with bemused disdain; Nazis, in contrast, they view with horrified disgust. Since the stigma against Communists is far weaker, the Communists manage to attract some vaguely normal adherents... or at least they used to. In contrast, the stigma against Nazis is so intense that you have to be virtually psychopathic to join. Once you send that signal, it's almost impossible to trust anything you say - even if you claim that you're no longer a Nazi.
As I say, I'm not sure I buy this 100% but it certainly is an interesting way of looking at it.