It's really hard not to leak information. This is just a guess, but if they see you doing something they don't like but can't decrypt, they can check their huge

It's really hard not to leak information. This is just a guess, but if they see you doing something they don't like but can't decrypt, they can check their huge PRISM database for your ip. Even if you do a good job at hiding who you are, communication outside of the encrypted channel you are using might reveal yourself. Also they have their hooks in Microsoft and Apple. It stands to reason that means a backdoor in each of the operating systems, as well. You'd have to really be paranoid to avoid this. Using an opensource OS find an open access point, with no surveillance cameras anywhere, use it only for your encrypted communication and nothing else, and then leave and never come back. And obviously, don't bring a cellphone with you. Considering this was top secret, I would bet that a lot of persons of interest didn't know about PRISM either, making it a lot harder to realize that they fucked up.