I stopped traveling in uniform when possible anyway both for PERSEC/OPSEC and because I felt awkward having people buy me drinks or meals when I'm on per diem and an officer who believes that he's well paid for what he does (it makes me happy when I see some young enlisted guy get theirs comped because they don't get paid well enough for what they do but I do get paid well enough for what I do).

If the military can justify having a bar at Al Udeid Air Base in Qatar where all the personnel there are supporting combat operations, I personally think having a drink on a commercial plane or at an airport bar where one may be on duty but that only duty is traveling from point A to point B is acceptable. The key is "drink" not "drunk".

Oddly other countries don't see this as an issue. Many European allies are known for having a drink at lunch. Heck the Germans and French even got booze in Afghanistan and apparently had a really nice set-up at Mazar-I-Sharif in northern Afghanistan. (US policy was no booze in Afghanistan while I was there but even that wasn't always the case).