I'll comprimise. I don't like the term entirely, but it'll do. Just like there are pro-gun-rights and pro-gambling-rights (and there used to be pro-slavery-rights).
Is it just for the sake of disagreeing with the religious? Is it because they see being anti-abortion-rights as being a strictly religious viewpoint? Are anti-abortion-rights atheists worried about fitting in? Is it just a coincidence?