I think you've oversimplified the legal situation here. Many states' laws do allow parents to give their children alcohol, but they may also allow the parents' practices in this regard to be considered as a factor in finding that they are unfit parents, which is a determination that should be made by a formal hearing, considering various evidence.

This may seem kind of bizarre, and perhaps the criteria in many states need to be made clearer and more objective, but it's not an inconsistent position. In many contexts the law allows parents to expose their kids to different kinds of risks, but if the overall amount of risk is considered too high, then the parents' fitness may be questioned. In the alcohol context, a judge might consider that letting children drink at meals in their parents' presence was not unfit parenting, but knowingly allowing the children to drink every day without any supervision was.