He's no longer active-military, he's a former Marine, so it isn't as if his service is even a factor here. He's a U.S. Citizen who posted some clearly disturbing things on Facebook. But it seems impossible to believe that a 30 day forced mental evaluation doesn't violate his constitutional rights. There are no specific threats made, although he does discuss a revolution.
Yes, he's a 9/11 truther, which I do consider a fringe kook position, but that's not evidence of mental illness. This detention is frightening.