Some might see it as being rather nitpicky, but to me it looks like a word created and designed by religious folks for religious purposes.
I think the same or something similar can go with related words such as "humanist."
To me it's like a way of saying that I don't opt into religion, by simply saying that I'm non-religious or not religious.
Once someone says that they're atheist, it seems to me like they've willingly - and perhaps unwittingly - chosen to step foot on the playing field of religion and religious belief systems. In everyday usage, it might not matter, but from what I've seen in debates between theists and atheists is a trend of the atheists apparently being duped into playing by the rules of the theists.
As an analogy, imagine being asked if you like a certain NFL football team; the question doesn't allow for a distinction between someone who's a fan of a different NFL football team, and someone else who simply isn't interested in NFL football in general. In reality, some (probably most) who identify as atheists would be analogous to those who simply aren't interested in NFL football in general, but the theists don't seem to make that distinction and treat them as being analogous to someone who's a fan of a different football team.