Since atheists have a common belief system and they are pretty organized why they do not consider themselves as a part of a another religion which does not have a god?
Because not playing sport is not a sport.
True! It couldn't be as simple as it is. If you are told to put in your religion, then you place "not applicable" because you don't believe in a religion. If you consider atheism as a religion then you defeat the purpose of an atheistic life. If you don't believe in a higher power then that is not a religion, you have an absence of religion. The above reply is kind of harsh, but it is true. You can't consider atheism as a religion like you won't consider not a playing a sport as a sport.