Yeah, the liberal audience already agrees so they won't stop to ask for sources. Communism = authoritarian by default.

In contrast, most communists have had their worldview shaken to the core. This lends itself to healthy skepticism about political/economic beliefs such that new beliefs require more justification. Which doesn't make all our political/economic beliefs correct--we don't all agree, after all--but we will at least point to some historical or theoretical evidence to back up our views.