(They got sick of Krauts taking potshots at them until they ran out of ammo.)
Wasn’t that their job?
The war was lost for Germany and everyone knew it. Most average German soldiers, when cornered, would surrender. Some would delay surrender until they had run out of ammunition. GI’s found this behaviour to be socially unacceptable and strictly enforced an unwritten moral code, known to the soldiers on both sides of the conflict. (The rules were different in the East.)
Fanatics like Hitlerjugend would never surrender. After Malmady, and to a lesser extent Dachau, surrender was not available to the SS, at least surrender to members of the U.S. Third Army.