Why should it be illegal? Why should we rely on the government to regulate personal behaviour?
it's not "personal behavior" (as stupid an excuse as that would be anyway)
they're not choosing, for themselves, to have it removed. their parents are making the decision before they could even possibly understand what the decision meant
it should be legal since it's common in religion and infringing on religious rights is wrong
stoning, despite being part of some religions, isn't legal either. the right to religious freedom doesn't mean you can do whatever you want to other people as long as your religion says you can