First off.
1. Who is the we? Everyone from Sikhs to Muslims to Christians all forbid the practice.
2. This is a good question. The standard used is that in practice it was not voluntary, and two, voluntary presumes that the person is in a correct state of mind.
It’s like the argument against suicide. We do not permit suicide in this culture even if some kill themselves anyways.
3. Religious freedoms are not absolute. Child sacrifice isn’t permitted, even though some religions participate in them. It’s a question of what is the higher value, doing what is right, or allowing everyone to do as they wish.
4. Freedom of religion is a right.
4.
Christianity is based upon human sacrifice. If Christians condemn human sacrifice on moral grounds, it is condemning not only itself as an immoral religion, but it is also condemning God of immorality. Not only did the Father offer up his Son has a sacrifice for the sins of the world, but he also commanded Abraham to sacrifice his son, Isaac. It is rather strange to praise Jesus as the sacrificial lamb of God, and then condemn another religion for practicing human sacrifices.
For either Muslims, Jews, or Christians to condemn religious sacrifices is hypocritical. I know little or nothing about Sikhs.
It is easy to believe that someone is not in his correct state of mind whenever he does something with which we disagree or do not understand. It is too easy to assume that conventionally accepted behavior is the only norm by which we determine what is acceptable religious practices. I could very easily see society condemning infant circumcision as child abuse. Just as I could see some counties in the deep South forbidding the use of wine for communion. If Christianity had practiced infant sacrifice throughout her history until today, it would be considered an acceptable religious practice and protected by law. The primary reason that certain religious practices are permitted and others condemned is a matter of cultural or conventional taste.