To the contrary, the court's statement that "FISA could not encroach on the Presidents constitutional power" is not only not "very ill-considered dictum", but it flows naturally from all the cases that preceded it, that held that the President had inherent constitutional authority to conduct warrantless searches to obtain foreign intelligence information. It flows naturally because of the long-established proposition that the Constitution trumps the law (FISA).
I smell word games. "Constitutional" can mean not prohibited by the Constitution, or it can mean actually granted by the Constitution. If it's merely not prohibited by the Constitution, then there's nothing saying Congress can't regulate it (or prohibit it).
It flows naturally because of the long-established proposition that the Constitution trumps the law
Only when they conflict. You wouldn't be assuming the very point in dispute, would you?