The reason is that a very large number of the soldiers are usually conscripts and fighting in war they were conscripted for. However, the islamo-freaks are soldiers who WILLINGLY went on their war on their own and are not affiliated with an official nation state!
That is why the islamo-freaks gain the title "enemy combatant".
This is why in some cases (allegedly) during WW-II they would take regular German soldiers prisoner, and captured SS officers would end up dead in the woods with a bullet in the backs of their heads.
I would agree with that. If the enemy combatant supports a known Nation State, with a declared head of state (who might conceivably sign a peace treaty) and if the enemy combatant wears a uniform of some kind when they fight, then they deserve to be treated with the honors of war under international law.
Terrorists? Torturing them to death upon capture should be mandatory. Napalming their villages should be mandatory. There are no rules. They are rogues, scoundrels and pirates. No quarter.