well they don't consider the fetus/baby to be a "person" because it's mental life is nonexistent or not as developed as an adult's. As the brilliant intellectuals in my philosophy dept would put it, a fetus is not a "normative agent." Of course this definition of "person" is just as arbitrary as anything, but hey when you're in power and your ability to whore around is on the line, you get to be as arbitrary as you want!
The murderer on death row, however, meets the liberal criteria of personhood.