There's all sorts of situations where it is legal for minors to drink alcohol.
Like for example, when accompanied by a parent in a bar in Wisconsin.
Anything else and it can be used to throw the book at you in a lot of ways.
I had a friend kicked out of our high school graduation because a teacher disliked him, convinced an officer to breathalyze him, and he blew a ~.02 if I'm remembering correctly. He'd had a beer with his parents at dinner before the ceremony, but by the time that was all sorted it was well after the ceremony had ended. Kid didn't walk the stage at his own graduation because of a power hungry teacher and a (legal) beer his parents gave him.
In most states in the US having any alcohol at all in your system or being in possession of it is very much a crime and the burden of proof is on you that it was consumed legally.
No bar owner wants to deal with nitpicking from law enforcement over whether or not the person with the minor was actually their parent or guardian, and serving someone that is obviously a minor is a big enough cultural taboo in the US that few if any business owners/managers are willing to do it.
One would've thought that maybe a place which stuck it's neck out and became known as 'the place to drink under 21' would get a lot of business, but I guess that could also be a potential PR disaster
[0]http://drinkingage.procon.org/view.resource.php?resourceID=0...
The activities listed - getting married, serving in the military, etc. are related to no longer being a minor. It is completely ridiculous that people who are not minors are barred from purchasing alcohol.