Isn't the whole point of "enemy combatant" that it allows people to be held in a state not governed by national, or international law or treaty? "PoW" has actual meaning, rules, treaties etc.

Or they could be treated like a criminal, but that would give them a lawyer, due process and so on.

I think that the US, and most western European governments still shy away from torturing PoWs or criminals. "Enemy combatants" however, are the dixie cups of modern policy. Fill 'em up, suck 'em dry, and throw the crumpled remains in the trash. All with a clear conscience.