Not to defend the abortionist, but the difference is one of voluntary versus involuntary...similar to how a person is free to smoke--but could still sue someone who poisons him with carcinogens dumped into his water.
I understand the point you make about the will of the mother, or lack thereof. Sort of like how did the woman get pregnant in the first place, by choice or by force. However, the person doing the killing of the fetus has a human life on his / her hands either way, no matter what our legal system says. That's the difference, or lack thereof, I am pointing out.