I agree that full US constitutional protections are not afforded to prisoners captured on the battlefield...but there are indeed some protections that are afforded to them by the constitution - those being any protections granted to them by any treaty signed by this government - which, according to the constitution, become the law of the land once signed.
The president has already stated that they deserve the protections of the Geneva Convention, and they clearly deserve the protection of the Convention Against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment which was adopted under president Raygun.