Because the federal government does not have a set date as to when a human life starts. No document says"Human life starts at conception so abortion is murder". Nothing says anything like that. Why do you think pro-lifers have their panties in a bunch?
I myself am pro-choice. I feel that a woman should have the freedom do do whatever the hell she wants to do with her body, as long as the second party (ie, the father) agrees with it. This means that, yes, even if the parents do not want the child, they should be free to abort it. I've been called a man who supports murder and much, much worse before, so I really don't care.
Pro-lifers hate abortion, but support the death penalty. Isn't that somewhat of a double standard?