Congress has declared war on Al Quaeda, its affiliates, its franchisees, its allies, and its sponsors, a vague and ill defined group, a nebulous category. This gives the president the legal authority to assassinate lots of people in lots of places on the basis of vague and secret evidence, some of them American citizens.
But it does not, however, give the president authority to assassinate abortion clinic bombers, or even Islamic terrorists who are not allied or affiliated to Al Quaeda, for example Hezbollah terrorists. Seems narrow enough to me. Congress could have, and arguably should have, declared war on Islam.
If anything government does is legal, if there is any legitimate purpose of government whatsoever, that purpose is making war. Congress has declared war in a completely constitutional fashion in response to an extraordinary act of aggression. Thus nothing the US government does could be more legal, more properly constitutional, than Obama ordering a drone strike on a US citizen on the basis of Obama’s reasonable and plausible suspicion that that US citizen is an Al Quaeda franchisee.
Just about everything the US government does is illegal and unconstitutional. Assassinating America’s enemies in accordance with a congressional declaration of war is one of the very few legal and constitutional things it does.
For this to be a legal precedent for launching a drone strike on me, Congress would need to declare war on “enemies of equality” or some such. Which it may well do, but I don’t think that assassinating real enemies in a real war makes this any more or less likely.