The low level guys guarding planes probably didn't have the autonomy to put their weapons away while guarding. This is SOP for a lot of military activities. Your weapon is out and held at the ready. Any other position invites liability as you can't get up to aim quickly enough in a firefight.
I'm not following. If you're a soldier that's how you hold your weapon when its out:
http://thumbs.dreamstime.com/z/us-soldier-17920177.jpg
The low level guys guarding planes probably didn't have the autonomy to put their weapons away while guarding. This is SOP for a lot of military activities. Your weapon is out and held at the ready. Any other position invites liability as you can't get up to aim quickly enough in a firefight.