Their leaders tell them that protesting the war makes you a traitor, and traitors are likely to vandalize monuments. Therefore they must show up and "protect" the war memorials during the anti-war protests.

http://home.comcast.net/~bfmuldrake/why.html

These guys are linked to the MIA movement, who bring some serious delusions of their own -- that some of their war buddies are still alive in Viet Nam and are prevented from coming home by a conspiracy of governments. The probable truth is that most of their friends were killed and the bodies disposed of in disrespectful ways, and a few others deserted.

Imagine the desperation of these people that they actually hatched this "excuse" to do what they do.
It's absolutely amazing the human ability to rationalize, even without beer!

As long as they're going to be there, they could at least clean the pigeon shit off of the statues. Out of respect for their national treasures.

It makes sense -- the Iran War Memorial is now in serious jeopardy.