Second, there are mechanisms for an American to lose his citizenship, and they're spelled out in every American passport. However, the wording say may lose citizenship. That means that someone looks at the case, and decides whether the schmuck is still a citizen or not.
Since passports come under the authority of the State Department, it's an executive function. There may be some obscure law that dictates how this citizenship board functions. Or maybe there isn't. Perhaps it's done under the department's general grant of authority to organize itself, just like it organizes the summer picnic committee.
In either case, if determination of citizenship was within the purview of the judicial branch, we would have heard of it already. That means the final decision on someone's status rests solely in the hands of the executive.
To prevent this from being arbitrary and capricious, one has to leave the US first, and then participate in the activities mentioned on the passport. When captured, someone in the DOJ will determine whether they still consider you a citizen, or an enemy combatant.
Personally, all of the clowns they're treating to a regular court trial seem like enemy combatants who do not conform to the Geneva Convention to me, and should just be disappeared into some black hole. But for whatever reason, they're being treated as citizens, although a bit wayward in what they did. I don't know why the government is doing it that way, but it sure seems legal and constitutional, even if it seems to be a tactical mistake.
I agree with you. Nonuniformed combatants (and/or war criminals) can properly and legally be executed summarily (or after military trial). I also agree that that would probably be a good idea in many cases (granting leniency, naturally, to those who prove of immediate use in providing intelligence to us).
At the very least, one or two such executions would make a point that this war is to be taken seriously. But these arguments seemed beyond the scope of what I was reacting to (Jacob Sullum's article in Reason Townhall).