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.
The state I'm currently in has no such exception. It is illegal for both the parent to supply and the child to consume alcohol at home. Both can be arrested.
Whenever I've gone to a beer hall in Germany the only super drunk people were American college students. It's really a shame, and you are absolutely right: We Americans are going the wrong way with this (and have been for decades).
My middle son turned 21 (legal drinking age here in the US) a couple of weeks ago. He never drank at all as a "kid." You couldn't give him a glass of wine at a holiday dinner, or a beer to watch a ball game or anything. Neither myself nor his mother were a part of his birthday celebration. I was honestly quite worried that he would be out with friends and have his first experience go horribly wrong. He has no idea how it will affect him. (Thankfully all turned out ok).
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.