Does it matter? It stopped the plot; just lather, rinse, and repeat, and POOF! No more terrorism, with the additional bonus of not spending crazy amounts of treasure spying on millions of innocent people.
Or it just means that they change their methods to ones you either can't track, or have a harder time tracking.
Such as? Can you provide an example of this method of communication that is fast, efficient, and somehow impervious to the reach of clandestine government agencies?
No, seriously, what are they going to do, use fucking carrier pigeons? Not likely.
As a result you don't know when or where they will strike.
9/11/2001, the underwear bomber, the shoe bomber, the Madrid subway bombing, the Boston bombing...
Doesn't seem TIA (which is the basic concept all this surveillance boils down to) has been doing much to stop terrorism as it stands, so that little excuse is getting a mite bit difficult to maintain, wouldn't you think? Hell, in at least 2 of the aforementioned attacks, the government had the fucking intel, and did nothing with it. If the ultimate goal of domestic spying is truly to curb attacks on American soil, don't you think they would, oh, I dunno, actually try to stop the attacks they know about?
I like your glib hand waving though. Could you do it a little faster? It's a bit warm in my room.
Well, hello Mr. Pot. Hard to recognize you with that black shit all over.