Interestingly, the practice is not obviously unconstitutional. As American citizens they almost certainly have due process rights, but the question of "what process is due" when U.S. soldiers have been trying to hunt you in Afghanistan for years is not a quick and easy one. "Due process" does not always require a trial, though in cases where the deprivation is life it almost always has meant that.
I wonder how you even get a test case for this. Who has standing to sue?