One of the goals of the military is to confuse the enemy and keep him off balance. At times you want to appear weaker than you are, sometimes stronger than reality. We bluffed Russia with the Star Wars program and won. Could this NSA thing be similar? Tell the world that we are listening to comunications that the enemy thought were protected. If they adapt and abandon working forms of comunication the end result is identical without actually having the program. It sounds too WW2 british so I doubt thats its the real story. It is fun to think about though.
Mmmm. Now, I'm trying to imagine some high-level al-Qaeda discussions during which the minutiae of FISA warrants figure prominently.
71-Hour Ahmed: "Sure, but a call from that international cell-phone would already be covered by FISA's 72-hour rule!"
Abu Death2Israel: "Then let's call from this other number, which conveniently falls through the cracks of the AUMF resolution!"
(71-Hour Ahmed and Abu Death2Israel, together): "Hahahahahahahah!!!!"