The reasoning behind this is that otherwise semi-private communication, such as email and encryption, reveals the existence of a conversation, and hence a relationship. The idea of using a social networking tool like Twitter is to hide both the conversation as well as proof of the relationship. Using something like lexical steganography, two malicious parties can communicate with each other in plain sight, without any connection linking them together. This is paramount when you're talking about building isolated terrorist cells; if one cell goes down, the bad guys don't want the government being able to connect it to other cells. Something like Twitter, which is mostly anonymous, is ideal for this kind of operation.