The question is: Did they want to thwart the attack?
One obvious possibility is that the system that has been erected simply needs fear to be able to continue to exist.
Let some attacks happen (and some Taliban prisons break) from time to time and people can be and will be manipulated via fear, so the powerful stay powerful and the rich become richer. Same old, same old...
Next stop: Middle Class Gone.
Then, people will have nothing to lose again and wake up. Next, we'll have riots and one day, the old system is dead and history will repeat itself once more. So much for the long term.
In wartime, it's not uncommon to not act on intel to stop an attack, so as to preserve its strategic value and save more lives later. For any insiders who truly believe in the NSA's mission (which is being generous), "let a few die today to preserve a system that will save thousands tomorrow" is not a big leap.
One obvious possibility is that the system that has been erected simply needs fear to be able to continue to exist.
Let some attacks happen (and some Taliban prisons break) from time to time and people can be and will be manipulated via fear, so the powerful stay powerful and the rich become richer. Same old, same old...
Next stop: Middle Class Gone.
Then, people will have nothing to lose again and wake up. Next, we'll have riots and one day, the old system is dead and history will repeat itself once more. So much for the long term.