Why not? So long as we are talking about a handful of US traitors, why abandon our usual legal procedures? Consider your own example. What if he says "I wasn't firing on US soldiers. They kidnapped me because they heard I was American, but didn't kill me because they found out I'm a Muslim." Sounds implausible to me. I'd vote guilty if that's the whole defense. But what if he can bring forth credible witnesses that support his story?
Something Bush said shortly after 9/11 has stuck with me, although he quickly forgot it. To paraphrase, he said we should go about our business and not let terrorists force us to change our way of life.
What would have happened to a German-American who sided with the Nazis during WWII, went to Germany to fight against our troops, entered into a plot to return to the US to kill a massive number of civilians in America, was captured while entering the country, and was carrying proof in his possession of his contacts with top Nazi leaders?
I think he would have been (should have been) executed as an enemy spy by a military tribunal, and that his capture would have been part of the war effort and not a civil matter.
so you want to have the US military travel with a criminal investigation team, just in case they might encounter US citizens on the battlefield? do we have to arraign the captured person within 48 hours too?
we are either at war with these people, or we aren't. either they are agents of foreign powers, or they aren't. we can't be at war with just some of them, while any US citizens who join the cause get treated like someone who just robbed $50 from a 7-11.