If I could provide a little history and context . . .
The image above was found here. There is little doubt that had the Taliban fighters taken the Marines and been given the leisure to deal with them, such beheadings would not be off the table, were the Marines alive or dead. American soldiers have run into such enemies before. This is one of the most famous photos of the Pacific War, 1941-1945.
Japanese officer beheading American soldier.
For their part, although there is no evidence that American soldiers and Marines returned the favor and beheaded live Japanese prisoners, what happened after they were dead was apparently fair game. Here are a few images of such instances:
Japanese head staked out in front of destroyed position.
Japanese head used as a trail marker.
Japanese head hung in a tree by an American serviceman.
Here's one mounted on a Navy vessel.
Original Caption: Defense worker N. Nickolson writes to her sweetheart thanking him for his letter and "souvenir." This skull of a Japanese soldier bears the inscription: "Here is a good Jap -- a dead one!"
Just a little history and context.