Specifically, why not treat them as pirates -- and hang them?
Because hanging is obsolete. They should be shot. Unless of course they are of any intelligence and/or propoganda value in which case such value should be extracted utilizing all necessary means, after which they should be shot (or hung, if you hopeless romantics prefer.)
EXCELLENT question.
On an earlier thread, a poster stated that the Geneva Conference only applied to uniformed soldiers. During WWII, any combatives caught out of uniform were summarily put up against a wall and shot.
Makes sense to me.