If I recall correctly the Marine brass announced early on that they weren't going to seek the death penalty.
For thirteen counts of murder? That's going to make the Brass look weak. If they're found guilty it'll be so...no kangaroo court for the USMC. Not executing them invites more of the same. Now understand, I don't know the specifics of the case. IF it's murky and maybe the "vics" were actually Jihadis THAT'S a different story! But to kill 13 innocent Iraqis -- maybe women and children...sends a bad message to let 'em live.