Bit fucked up isn't it?
So in the US its perfectly ok for a 19 year old to serve and maybe even die for their country but it isn't legal to serve this same 19 year old a beer?
I never cared for this rule.. Even when I was serving in the Army, my first duty assignment was in a different country, the drinking age was 18 so I was good. I do feel you should be able to drink if you can show a military ID no matter where you are in the States.
One problem though I see is that some young people that never drank alcohol before joining the military causes issues with Alcoholism.