Let's say they're not in the same location. They can send encrypted email. It doesn't even have to be email. They can paint an encrypted message on the side of

Let's say they're not in the same location. They can send encrypted email. It doesn't even have to be email. They can paint an encrypted message on the side of a fleet of commercial airliners and read it out loud during a football broadcast. As long as there's a way to encrypt a message, there's a way to send it to someone who has the decryption key. Demanding that no private communication is available to terrorists requires absolute surveillance. Of course, some people are going to get sloppy and reveal secrets over the phone or other unencrypted or under-encrypted channels, but that's not the subject at hand. The stated goal was to ensure that terrorists have no means of private communication, which has some pretty awful consequences.