The reason is that some people believe a fetus has human rights and some people don't. This is a moral question that depends on several philosophical or theological assumptions. If you believe human beings are just another part of the food chain, abortion, infanticide, or even euthanasia might not be an ethical impediment to utilitarian arguments for population control. On the other hand, if your religious beliefs impel the conclusion that we have God-created souls which are present in a fetus during pregnancy, it logically follows that abortion is an act of homicide. The first American Principle solves this problem nicely by allowing for disagreement based on different moral conclusions.
Freedom of religion means being free from other people's religious conclusions.
Regardless that you view abortion as murder, you understand that there is no secular consensus on this and leave the punishment for abortion, if any, to God's judgment, not man's. I sincerely don't understand how anyone expects public schools not to be a source of never-ending political strife. Parents want their children to be taught the values they themselves hold dearest. If your Christian values are that God is the source of moral law and Jesus the source of human salvation, you want the Lord's Prayer recited in class each morning, the Ten Commandments on the classroom wall, and an Easter Pageant each spring. If you're Jewish, keep the Ten Commandments but get rid of the Lord's Prayer and the Easter Pageant.