Actually, it largely does. POW's cannot be summarily executed. Saboteurs and other combatants disguised as civilians can be. It's true that a given combatant doesn't forfeit protections based on the violation of other combatants on his side (including his superiors), but that's not what I'm refering to.
Just to clear up a common misconception, lets look at US Military Regulations on this subject.
Quoting from 1-6, it's clear that a tribunal is required whenever the prisoner claims to be entitled to POW status.
b. A competent tribunal shall determine the status of any person
not appearing to be entitled to prisoner of war status who has
committed a belligerent act or has engaged in hostile activities in
aid of enemy armed forces, and who asserts that he or she is entitled
to treatment as a prisoner of war, or concerning whom any doubt of
a like nature exists.
But even when the tribunal rules the prisoner is not a POW, we still can't take them out and shoot them.
g. Persons who have been determined by a competent tribunal
not to be entitled to prisoner of war status may not be executed,
imprisoned, or otherwise penalized without further proceedings to
determine what acts they have committed and what penalty should
be imposed.
Back in 1-5 it spells out how these proceedings should be conducted.
The punishment of EPW, CI and RP known to have, or suspected of having, committed serious offenses will be administered IAW due process of law and under legally constituted authority per the GPW, GC, the Uniform Code of Military Justice and the Manual for Courts Martial.
At least on paper, the US is a civilized nation and does not condone summary execution.