Well, they tout altruism, but they don't actually practice it. It's all fake; if they were true altruists, they wouldn't need to argue all day about people needing to share their stuff. In altruism, there is the concept of not personally benefiting or not requiring reciprocity.

But the demagogues endlessly promoting communist ideologies definitely benefit from it by appearing morally superior and getting resources for no valuable work in exchange.

The GPL, shows that, actually, they are not really ok with the no reciprocity part.

There are very few truly altruistic individuals, and their defining characteristic is that they just do the good stuff instead of endlessly talking about it for brownie points. Basically the complete reverse of communists (and everyone far left in general).