The State Department says that there are "as many as 100" Americans fighting with various extremist groups in the region, including but not limited to ISIS. So although McCain was killed in fighting with the Free Syrian Army, it's only a matter of time before a U.S. airstrike takes out one of these U.S. citizens, and only a matter of several more seconds before someone begins to complain loudly about the denial of their due process rights.
It might not be the same people, since folks like Sen. Rand Paul (Ky.) aren't exactly known for their consistency in this matter, but surely, someone will draw the same conclusions that were made about the killing of Anwar al Awlaki and Samir Khan, two U.S. citizens who were killed in a targeted drone strike. As the President and everyone else in the media never tire of reminding us, ISIS are terrorists, not a legitimate enemy fighting force, and so those 100 or so U.S. citizens, like Awlaki, are nothing more than terror suspects. Shouldn't they be afforded the same high-decibel outrage that he was?
I've always had two major problems with the outrage surrounding the killing of Awlaki, the first being the premise that it's not okay to kill him because he was born in New Mexico, but if he'd been popped out in Manitoba, all other things being equal, then kill away. If the idea is that we should't kill him because he might be innocent, because he's presumed innocent, then we shouldn't be killing anyone in this manner.
But the second, most easily-solvable problem is that angst over drone strikes, right or left, is always used as a way to club President Obama, rather than to actually fix the problem. If we don't like the laws that allow this sort of killing, then we ought to change them. In al Awlaki's case, that would be the AUMF, which Congress has shown little interest in altering, let alone repealing.
With regard to the current situation in Iraq and Syria, the President is acting under the authority of the War Powers Act, which theoretically allows Congress to weigh in on the decision to use force in the region, but which, in practice, allows them to punt back to the President and continue sniping from the sidelines. If Congress wanted to force the President, any president, to seek congressional authorization to continue military action, they could do it, but they like it just fine this way.