I disagree.
How you arrive there is important. The argument being made by SeekAndFind, that FISA authorizes the President to issue warrantless searches against US Persons, is, in two words: Bull $hit.
It does no such thing.
Reading the entire statute, there is no grant of authority for warrantless surveillance unless the communication is entirely among or between foreign entities, or "there is no substantial chance" that the surveillance will involve US Persons.
[And, by the way, it is not clear on Constitutional grounds that the President doesn't already have such authority. IIRC The very first ruling by the FISA court contains dicta to the effect that the court is not presuming on itself the authority to interfere with the President's extraterritorial national security authority.]
The larger question is whether the President has inherent authority under the Constitution to issue warrantless searches in matters of national security. I do not personally agree with your conclusion that this is presumptive in the authority of the Executive. Had the Founders intended such authority -- and they most certainly did not given both the list of abuses publicly laid out just 11 years earlier and the commentary on the Executive in The Federalist -- it vanished on the day the Fourth Amendment was ratified.
Allowing the President, on his own authority to both decide what constitutes a national security concern involving US Persons and to warrant searches and seizures of same makes a mockery of both the Fourth Amendment and the Separation of Powers.
That was addressed in the Keith case. The president's power is in foreign intelligence, which may or may not involve national security. Aldrich Ames case is one that involves a US person, no warrant.
It's been awhile since I studied the statute in detail, but I do recall the argument that the statute itself steps on the president's toes - not for every case, but for some cases.
My general point is that Obama's actions, while maybe on "iffy" legal grounds, are at the same time on political quicksand. It benefits Obama to have the argument be one of legality.