Drinking an unsupervised beer still seems less dangerous than taking a supervised bullet.

Not really.

"We are going to trust you to go into war - a stressful situation - and know who to kill and who not to kill, if need be... but no, we definitely won't let you sit at home and drink a beer, even if it is with your parents. That is criminal"

It is pretending to treat someone like a mature adult, with all of the responsibilities, but refusing to allow beer and bars, which are common recreational and social habits in the culture.

Voting and drinking ages used to both be 21. Notably, around 1969-1971 during the height of the Vietnam draft, 30 states lowered their drinking and voting ages to 18. Most of these went back to a drinking age of 21 by the early 1980s, but voting remained at 18. It was directly due to pressure from the anti-war movement.