Is it just for the sake of disagreeing with the religious? Is it because they see being anti-abortion as being a strictly religious viewpoint? Are anti-abortion atheists worried about fitting in? Is it just a coincidence?
Why are there so many folders devoted to abortion? Since som have been around for quite awhile, you can get the answer to your questions just by reading the discussions over the ethics of abortion policies.
I think that the best place to consolidate these discussions is the "Pro-Life" folder in the "Politics" drawer.
I dont like the way the question is framed... Pro abortion gives me a vision of forced abortions and thats just disturbing. Most Xtians are anti-choice because they are hung up on the idea of an immortal soul. as an atheist i know that there is no evidence for this and even research disproving the idea. As an atheist I can (try to) make a rational decision based on available evidence. Until a certain time in the pregnancy (the biologists can help out here) it is just a clump of cells. When it develops a brain and is able to feel sensation and stimulus then it should be considered a viable person. I am not saying that abortion is great and we should all get one, I am saying that taking the choice away from the mother is more detrimental to society than providing a clean, safe and sterile place.
Tough cookies. Pro-abortion does not mean forced abortions anymore than pro-gambling means forced gambling, or pro-gun means you think guns are great and everyone should have one. Pro-gambling is consistent with the opinion that gambling is horrible and no one should gamble.
"Pro-choice" is a meaningless and vague euphemism meant to distract from the actual issue. By your logic, "pro-choice" could equally be applied to the pro-gambling crowd (choice to gamble), or those opposed to company mergers (choice of product) or traffic laws (choice to drive backwards on the wrong side of the road). Some pro-slavery people thought it should be left up to each white man to decide whether or not to own a slave based on his own moral compass (http://www.forerunner.com/forerunner/X0465_Pro-Choice_Rationale.html); they would also be "pro-choice".