This is both a legal and a moral concern.  First of all, when a US citizen takes up arms to fight for a terror group against the US, it would seem that they have given up their citizenship rights to a civil trial. Criminals in the US are often shot in the effort to apprehend and we accept that as a necessary process. Of course in the drone situation, they are not given a chance to surrender and become a prisoner of war.  A drone kill is questionable when there is not sufficient knowledge to know if the US citizen is truly guilty of treason. I have not heard evidence that there is always proof of such guilt.  That, in my opinion, is a real problem for the President. Another piece of this is the President's objection to poor treatment of prisoners. How can he kill Americans and justify this killing when he is worried about mistreatment? How is death more defensible?