As long as any belief includes a sense of it's own superiority and ufallability in it's dogmatic base, it can be used to attack any other belief.
Well that's really the crux of the issue, isn't it. Atheism does not have that. Religions, at least of the Abrahamic variety, claim to ultimately derive their dogma from an infallible, infinitely wise god. Atheism, on the other hand, only says that as far as we know, through our limited human understanding, there's no evidence for a god.
The difference between the way atheism was used in the Soviet Union and the way religion is used is in the dishonesty. Those who manipulate people with religion claim to do so for the deity, while in reality pursuing their own goals.
Marx, on the other hand, wrote that "the abolition of religion as the illusory happiness of the people is the demand for their real happiness", which was of course to be achieved through Communism. He doesn't urge people to do stuff in the name of atheism, on the contrary, he openly says that atheism is merely a means to an end, a necessary prerequisite for the true goal of establishing a classless society. Ergo, atheism was enforced in the name of communism, not the other way around.
Addendum: Eh blast, Leafsnail beat me to it.