I'm not sure it's necessarily a discipline thing. If you look at the link below, you've got a SSgt with his helmet (and flak jacket) undone, in front of a captain and a four star general no less. (It's in the caption.)
http://www.latinamericanstudies.org/...grenada-10.jpg
Maybe it's a generational thing? That SSgt looks like he's been around a while.