One word: FREEDOM.
It is the thing which defeated fascism. It is the thing which defeated communism. It is the thing which will defeat terrorism. It showed on the purple digits of the people of Iraq when they tasted it for the first time in their lives. It showed on the faces of the people of Afghanistan when they tasted it for the first time in their lives. Nothing is sweeter. Nothing is more fulfilling. Nothing can replace it or replicate it. Nothing is more worth fighting for. Nothing is more worth sacrificing for.
Why did her son die? So his mother could see the results of his sacrifice on the faces of formerly oppressed people. So she could know that by his sacrifice, the world was made a little bit safer from the hydra of terrorism.
Why is his mother doing this now? She has listened to seemingly sympathetic voices and has not recognized them as the "America is Evil" cabal who took delight in seeing her son's flag-draped coffin. They revel in American body counts. They rejoice in American casualties. She has become a pawn to the very people who would disparage her son if he were still alive with placards saying they support the troops when they kill each other.
Alas, she has become a pitiable tool. All too willingly.