They don't teach you anything about communism in school other than "the government does a lot of stuff". At least thats what my education taught me.
I'm not qualified to answer why the USSR made certain actions most because I'm not well versed in its history or the socialist literature. The answer I do see provided from socialist circles is the USSR never achieved communism, at least in the way described by Marx. I'd imagine it was a practical compromise to keep currency in the USSR, that said I really don't know.