They do. Because they are uneducated or brainwashed and never stop to consider if their interpretation of a certain thing is actually correct.

That is basically Bordiga's argument (ok I am oversimplifying it a bit, but they state in "Dialogue with Stalin" that "commodity production, including private property, is neither “natural” nor, as the bourgeois claims, permanent and eternal. The late appearance of commodity production (the system of commodity production, as Stalin says) and its existence on the sidelines of other modes of production serve Marx to show that commodity production, after it has become universal, just after the spread of the capitalist production system, must go down with it." This has some anarchist energy right here).

These people never think that far. They heard the meme definition of the communist stage of development somewhere (stateless, classless, moneyless), see the USSR getting called communist by liberal media (never noticing the contradiction to the name of the Union...). Never heard of the transitional stage of socialism they then come to the eronious conclusion that USSR does not equal a stateless, classless and moneyless society, thus it can only the only other thing! Capitalism!