Good question. You should have brought it up when the legislation was passed in September 2001. Here's the applicable language from the Authorization to Use Military Forced (AUMF):
(a) IN GENERAL- That the President is authorized to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons.
You see the "he determines?" The Obama administration didn't make that up, because it's currently valid law. And it will be valid law until it is defeated in court or repealed. Section (b) says the AUMF complies with the War Powers Act which is complete BS, and the AUMF in total is an over delegation of congressional power a la Chadha.
But I don't make the rules.