I think the ideal situation in all cases would be taking a suspect, bringing them to trial and awarding an appropriate sentence if they are found guilty. The difference between a US national and a foreign national is a US national has a real chance of actually being seen in court. Though it would be infinitely preferable to do so, I can't imagine the logistical complexity of trying to defeat a terror organisation by individually capturing and trying each suspected member. It's a new kind of war, and not one with a set of engagement rules like those that governed more "traditional" inter-state warfare.