Check your sources.

The National Defense Authorization Act (NDAA), under “Subtitle D - Detainee Matters”:

IN GENERAL.—Congress affirms that the authority of the President to use all necessary and appropriate force pursuant to the Authorization for Use of Military Force (Public Law 107–40) includes the authority for the Armed Forces of the United States to detain covered persons (as defined in subsection (b)) pending disposition under the law of war.

What is a covered person, as defined in subsection (b)?

(b) COVERED PERSONS.—A covered person under this section is any person as follows:

(1) A person who planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored those responsible for those attacks.

(2) A person who was a part of or substantially supported al-Qaeda, the Taliban, or associated forces that are engaged in hostilities against the United States or its coalition partners, including any person who has committed a belligerent act or has directly supported such hostilities in aid of such enemy forces.

Now, tell me where it gives President Obama the power to imprison U.S. citizens on a whim?