Why are U.S. Army MPs patrolling the concourses at Boston Int'l Airport?

[Pacifist Patriot]

When did this happen? I've flown in and out of Boston, Newark, St. Louis, Atlanta, Orlando...since 9/11. This past week was the first time I've noticed armed military personnel roaming the hallways of an American airport. When and why did Boston start this patrol?

I'd removed my shoes, stripped off my shirt (over a tank top so it constituted a jacket I suppose), emptied my pockets, taken off my watch, placed everything on the conveyor belt through the x-ray machine, dutifully avoided any liquids or gels in my position and presented my picture i.d. and boarding pass when I entered the security line and then AGAIN ten feet later as I was going through the metal detector.

What exactly did the MPs think any of us were going to do while waiting for the boarding call?