Taking a knee at the grave of a fallen soldier isn't a protest, it's a sign of respect and sorrow that a fellow service member has fallen.
You DO realize that when they have a memorial ceremony for fallen soldiers on the battlefield that they put up their boots, rifle and helmet as a way to remember them, and a lot of their comrades will go up to the display, take a knee, say a silent prayer, then stand and go on their way.
But, if you had actually ever served in the military, you might have understood that.