Well, a zygote is a developing human being while a placenta is clearly not. You don't have to believe in a soul to think that all human beings -- regardless of their stage of development -- merit legal protection.
Giving the 'undifferentiated mass of cells is clearly a human being' argument even a slight push very quickly devolves into absurdity.
Some animals have a phenomenon called parthenogenesis which allows an unfertilized egg to develop into a living organism. This does not naturally happen in humans. (And I doubt it happens unnaturally.)
So why again should humans be punished when it happens all the time anyway "by design"?
In that case, pro-lifers should have no issues with a pregnant woman asking her doctor to remove her placenta - as long as they don't touch the zygote.