It's a point drilled into the heads of all U.S. soldiers, so it's hardly subtle. And technical though it may be, it is a core definition.
Yes, they could have been shot, but they weren't. They were taken into custody. World of difference.
Nonetheless, they fall into an intentianal treaty black-hole. They are specifically and explicitly excluded from protections of the Geneva conventions. This is done so as to protect noncombatants from the effects of legitimate military actions.
By the Geneva Conventions, rather than liberal la-la land, the people who are conducting combat without being clearly identified as a military force are responsible for the harm that comes as a side-effect of taking them out.
A similar example: Using an occupied protected area for combat operations or munitions storage is illegal under the conventions. If another military comes and blows the whole mosque or school, or hospital to bits, those who occupied the facility, as well as any who had to do with ordering the facility to be used in that matter are the only ones guilty of a war-crime - not the ones who blew it up. Like in OKC, if attacking that building was an acto of war, killing all of the kids would have been a war-crime by our government for placing them within a legitimate target, not the attacker for having killed them.
Each of these examples, as well as the "technicality" is a form of using non-combatants or protected places as a shield - which legally, is the act that endangers the non-combatants. That crime renders pretty near all treaty protections void for those involved. Conducting War without a uniform uses the common people in the area as a shield.