Wounded or maimed troops take up more resources than dead ones. That's why modern AP land mines are designed to maim rather than kill, and why unexploded cluster munitions aren't designed to break down in the environment- they become mines.
Caring for wounded comrades is a lot more difficult than leaving dead bodies behind... and wounded insurgents may well seek additional care down the road leading to the possibility of capture and interrogation...
The Red Cross doesn't really care about any of that, anyway... they're pretty much non-partisan wrt saving people's lives... and today's Afghan insurgent could easily become tomorrow's peaceful civilian, if he had a govt he could respect and support- you know, one that wasn't rotten to its core...