This all assumes that at the time we legally believed that the Geneva Conventions covered the people who were captured.
Protocol III of the Geneva Convention is what would cover these groups, but the United States never signed that part and objected to it, thus making the US not bound by it.
Of course you can hold people to a standard of legality that did not exist at the time if you would like, but it makes no difference in actual reality.
This is not to excuse a few instances where we did cross the line, but those instances have been dealt with already, and the soldiers who did it prosecuted.