Are they usually armed?
…and of course, the presidential dog also gets armed protection!
In cruising around for pics, it seems that sometimes they do, and sometimes they don’t. Often times, the guard near the forward part of the aircraft is unarmed, and the guard to the rear is armed.
Perhaps it depends on a number of different factors as to what the protocol is.
I would also guess the location has a lot to do with it, landing/taking off from the White House is one thing, but when landing at a commercial airport or other restricted area they may wish to be more discreet with the weapons.