Manslaughter or Abortion?

Here in Tucson, a Mexican national was sentenced to 23 years in prison. He was convicted of crimes relating to an accident in which he killed an 8 month old fetus/unborn child and seriously injured the mother. The story was in the Tucson Citizen, "Judge Howard Fell sentenced Carlos A. Frasquillo to 10.5 years for manslaughter in the April 16 death of the fetus and a consecutive 7.5 years for aggravated assault on the mother, who suffered severe injuries..." My question is why the mother could have killed the very same fetus, by choice, and had no legal consequences. If it's considered life when someone else kills it, why is it not considered life when the mother kills it? This baffles me. Any thoughts? Link to the story below. http://www.tucsoncitizen.com/ss/fromcomments/108117.php

-A Vagabond Thought