Criminals are generally not all that smart, and suicidal terrorists are especially not-smart. However, most of the suicidal religious lot have worked out that the secure way to communicate is by meeting up face to face.
Once an attack is in progress, comms don't really need to be secure; if you assume that the security services aren't on the ball enough to know who all the participants are (a good bet if your little jihadi plot has gotten to live state), then you can also assume that they aren't going to understand the comms chatter quickly enough to make any difference.
That was the assumption the French terrorists made: they used completely unencrypted SMS to start and coordinate their attacks, and over that short time scale it worked.
Biting the hand that feeds IT
