Disgraceful, but not inexplicable. I'd wager that being out there in the sticks, seeing your buddies ripped to shreds by IED's, mortars, and snipers would leave anyone itching for revenge. It doesn't make it right though; the fight was over, and be they military, civilian, or space alien, it doesn't matter, peeing on dead bodies is not only wrong, but also rather creepy. Those guys need some mental help- getting off by urinating on a corpse with your buddies isn't a sign of a healthy mind.
-Penguin