calling atheism a religion is like calling abstinence a love position
or not playing baseball a sport
or not smoking a habit
etc
Those are stupid examples. My point is, anyone you ask will have a set of beliefs. Not everyone plays sports. Not everyone has love. And a habit is something you do by action due to your brain becoming accustomed to repeating that action, usually without you thinking about it first.
Anyone who is old enough to think for themselves basically has a set of beliefs, whether those beliefs are morals or the idea of divine being(s).
By calling someone atheist, it is implied that they do not follow any religions, but in that, it can be considered its own religion.
Now, let's stop before we get into the war of the words.
Atheist, considering Buddhism
I'm atheist, but I think that some of the philosophical values of Buddhism are good. Actually, I consider it more like a philosophy than a religion.