Uh, they do, actually. The belief is that the USSR was attempting to build communism.
I reject this because they didn't implement any communist policies, even Lenin himself said what they were doing was state capitalism.
The tankie defense is that they were doing it to later establish communism, but nobody who knows what they're talking about actually believes the USSR achieved socialism.
They even refused to help Yugoslavia after they implemented an actually socialist policy (workplace democracy).
The situation with the USSR was rather clearly that an authoritarian dictatorship wanted to call themselves a utopia so they said they were communist.
I don't understand why people give the USSR the benefit of the doubt and assume they were actually trying to do something good.