Maybe I'm just not in the right areas of the airport, but I can never recollect seeing anyone armed beyond a small pistol in the airport. Which airport was this?
Knowing how government work it would not surprise me at all to read that each airport now has TSA officers with SWAT training and they have all kinds of weaponry stashed in a locker somewhere.
Give it a few years, they will be.
Less an institutional approach to security, more an example of institutional panic.
The NG is fine with passing out guns - they probably break them out one weekend a month and dust them off.[1] But passing out ammo - that's a serious step. Some officers freak the heck out when they think about troops having actual live bullets in their magazines.
In the 1980s era Marines, on barracks duty, I actually rolled out to a for-real no-kidding 'we're being shot at' incident with a rifle, a radio, a flashlight and no ammunition.
The reasons _why_ are complicated (and this isn't the place for a sea story) but it boils down to 'the troops weren't trusted with bullets - they might shoot someone'.
[1] I understand this has changed, for the better, since 9/11. Now they break them out and _clean_ them on drill weekend.
I DID see something similar in a British one, though. Soldiers (not guards or police) patrolling the terminal with (to my eyes) very large guns. They also seemed to be in "spread out"-formation, keeping their distance from each other. A measure I would have thought was reserved where more caution was needed, e.g. street patrolling in a war-zone.
A rather unnerving sight, actually.
But I did see them in Frankfurt and CDG _before_ 9/11. So in some countries it's quite routine to have (para)military personnel guarding airports. In the Us it seems to be the exception. I haven't seen any recently.
If they are doing a drill or the threat level goes up, they are armed with rifles, and the cops have sub-machine guns. Fortunately, they are unloaded, so you're less likely to have an incident where they start shooting at some deranged homeless guy. Being deafened by some idiot shooting a rifle down there would suck.