I guess they are using the authority under AUMF as a separate justification than the one he has under the Constitution.
The administration has basically raised three arguments. The only one that holds water under intense pressure is the inherent authority argument, countered with the 4th amendment.
The "legal under FISA" argument, which Gonzales is making, stinks to high heaven. It is a poor argument, and I am surprized it is even made.
THe "legal under AUMF" argument is fair, but has a number of foraward-looking weaknesses that I have pointed out above.