For most of the last dozen years, the AUMF provided adequate domestic legal cover for U.S. drone strikes, since nearly all the individuals targeted were believed to be senior Taliban or al Qaeda operatives. Of course, U.S. drone strikes could be criticized on other grounds -- as strategically foolish, or as lacking in transparency and protections against abuse -- but strictly from a legislative perspective, most of the early U.S. drone strikes appeared comfortably within the scope of the congressionally-granted authority to use force.
But this has changed in the last few years. The 9/11 attacks are receding into the past, the war in Iraq -- which also had its own special AUMF -- is over, the war in Afghanistan is winding down, and al Qaeda has been significantly weakened. As a result, the United States is running out of "nations, organizations, or persons" that can plausibly be viewed as complicit in the 9/11 attacks.
This does not mean we've run out of terrorists. The world, it turns out, offers a nearly inexhaustible supply of people who don't much like the United States. Some subset of those people self-identify with the distorted brand of Islam favored by al Qaeda and the Taliban, and a further subset are prepared to use violence to further their ends. Thanks in part to heavy-handed post-9/11 U.S. policy, al Qaeda became both a franchise and an inspiration to aspiring terrorists all around the globe. As a result, it's now possible to find plenty of unsavory groups that might be inclined to do harm to the United States, had they but world enough and time.
When you're not near the terrorist you're targeting, target the terrorist you're near
Once established, bureaucracies take on lives of their own. Having built up an elaborate military and paramilitary apparatus to go after the 9/11 perpetrators, the U.S. government now finds it hard to scale back down, even though many of the groups now being identified as threats don't fall clearly under the AUMF's umbrella -- and many don't pose a significant danger to the United States. It's not entirely clear, for instance, how meaningful the ties are between Somalia's al-Shabab and al Qaeda, and there's little reason to view al-Shabab as complicit in the 9/11 attacks. To the extent that the 2001 AUMF was focused on prevention, there's also little clear reason to focus on al-Shabab, which has historically had only local and regional ambitions. Granting that al-Shabab is a thoroughly nasty organization, is there truly a significant likelihood that it poses an imminent, non-trivial threat to the United States?
"When you're not near the girl you love, love the girl you're near," sang Frank Sinatra. The U.S. government seems to have its own variant: When you're not near the terrorist you're supposed to target, target the terrorist you're near. To accommodate this desire, both the Bush and Obama administrations have had to gradually stretch the AUMF's language to accommodate an ever-widening range of potential targets, ever more attenuated from the 9/11 perpetrators.
The shift has been subtle, and for the most part Congress has aided and abetted it. In the 2006 and 2009 Military Commissions Acts, for instance, Congress gave military commissions jurisdiction over individuals who are "part of forces associated with al Qaeda or the Taliban," along with "those who purposefully and materially support such forces in hostilities against U.S. Coalition partners." This allowed the Bush and then the Obama administration to argue that in the original 2001 AUMF, Congress must have implicitly authorized the use of force against al Qaeda and Taliban "associated forces." That is, if Congress considers it appropriate for U.S. military commissions to have jurisdiction over al Qaeda and Taliban associates, Congress must believe the executive branch has the authority to detain such associates, and the authority to detain must stem from the authority to use force. This suggests that Congress must believe the AUMF should be read in the context of traditional law-of-war authorities, which include the implied authority to use force against (or detain) both the declared enemy and the enemy's "co-belligerents" or "associated forces."
By 2009, the Obama administration was arguing in court that, at least when it comes to detention, the AUMF implicitly authorizes the president "to detain persons who were part of, or substantially supported, Taliban or al Qaeda forces or associated forces that are engaged in hostilities against the United States or its coalition partners" (my emphasis). Note how far this has shifted from the original language of the AUMF: The focus is no longer merely on those who were directly complicit in the 9/11 attacks, but on a far broader category of individuals. This broadened understanding of executive detention authority was later given the congressional nod in the 2012 National Defense Authorization Act, which used virtually identical language.