You're right, there are atheist organizations. Those groups have an agenda but are on the fringe, similar to PETA, the NRA, Focus on the Family, etc. Most people who identify as atheist don't belong to any such group or have any agenda. They just don't believe in god.
I'm not sure why they have to identify themselves as being "atheists" or anything at all for that matter. If you don't believe in something, you simply don't believe in it.
I don't believe in ghosts, but I don't go out looking to find and identify myself as part of a larger group of people who also don't believe in ghosts. It's completely pointless.
I'm glad you can at least admit some of these people DO have agendas. However, I think the other groups you claim are "on the fringe" are not anywhere close to the fringe of society and actually have far more of a legitimate claim to do what they do than any of these crazy atheist groups.