I guess there's an exception to everything - I carry legitimately when hunting. Of course, I have to call the MP desk or Sportsmen's Center to sign out and back in to hunt; so they know they have an armed civilian on Post.
Come to think of it, I also carry when I come on Post to shoot at the Plinker Range. Civilians can carry for legitimate purposes. It's a shame that self-defense isn't one of those purposes.
I don't think the Founders had this in mind when they wrote the Constitution and the 2A.