We find it completely palatable to kill a murderer in our system of retributive justice. We find "pulling the plug" to be fine because it's "in his/her best interest."
Obviously, the prohibition against murder is more emotional than cerebral. Thus, since the mother doesn't want the baby, thus will not miss it and the fetus does not have a consciousness to know that it is going to die, then it can't be murder.
Furthermore, if we'll accept the "best interest" reason, then any child of my siring is, by far, better off being aborted at this moment. I can barely afford myself, let alone support another 2 human beings.