I can't think of a single sensible reason to pretend that atheism is a religion, and that includes tax exemption.
Getting away with anything should not be among our intentions.
I also would not call atheism a belief, at least not in the same sense that religion involves belief. It's the
default position. Someone makes a claim that some supernatural entity exists - we choose not to believe them because there is no evidence. We're not adopting an alternative position - it's not black vs. white - we're just disregarding the assertion because it's unsupported. This logic is applicable in any other situation, it's not our fault that theists insist this particular situation demands special treatment. It doesn't.
I'd be willing to bet that the average Christian feels he has no reason to believe in Zeus and that it would be silly to even consider it seriously, let alone
identify his non-belief when describing himself to others.
Does not believing in Zeus need to be defined as a distinct belief system?
Only if you're playing games with words and trying find a loophole for argument's sake. Or if you're trying to secure tax exempt status....
Atheism
is not a religion.
I'm starting to sound like a broken record with that last line.