They can really arrested you for drinking as a minor? In here a bar can get into trouble if they sell you a drink, but your only trouble are parents if they ever find out. And I remember that 18-19 years old Americans I knew indeed used to drink (and get wasted too).
As always in the U.S., it depends on the state and city where you live. But I'm pretty positive there are cities where it's actually illegal for a person to be drinking underage. Knowing some of our cities, mere possession of an open container by itself might be enough.
However as you note, that hasn't really stopped teenage drinking...
It's worse than that - entire states have adopted the concept of a minor in possession by consumption - if a minor is behaving in a way that leads police to believe they are intoxicated, that is enough to charge them with being in possession of alcohol.
When I was in college, some dorm buddies of mine got caught drinking on a public beach at night, were arrested, and charged with "minor in possession." I'm pretty sure this was a state law, and I'd be surprised if it wasn't widespread.
