I bring this up since many people do in fact think that atheism is a religion.
Many people also think that the earth was created in 6 days about 10,000 years ago. They're all morons.
Generally speaking, atheism is a religion like baldness is a hair color.
More specifically, atheism is not a religion for the same reason that theism is not a religion: they are
attributes of worldviews. Clearly, Islam an Christianity are not the same religion, but they are both theistic. Secular Humansim and Mahayana Buddhism are not the same worldview, but both are atheistic.
Having said all that, when talking about atheism and contrasting it with other worldviews, the general topic is "religion" since that is something easy for others to recognize. It doesn't follow from the fact the subject of atheism pertains directly to religious conversation that atheism is a religion.