What bazillion is trying to get across is that people are people. They tend towards that which is most convenient and least painful. Sure, HN could come up with the worlds most secure, least detectable, most effective means of communicating with anyone, anywhere at anytime. It will also be a PITA for people to use. Hence, when your mind is occupied with blowing people up or kidnapping someone or hijacking a plane or stealing state secrets, you end up not spending a lot of time on the aforementioned Perfect Comms Path.. you use gmail and pgp the contents (maybe)
I don't think what you say is true for serious terrorists. If they were that naive, they would be easy targets.
It seems more likely that the NSA isn't just hunting hardcore terrorist cells. Terrorists don't act in a vacuum. They are the extreme end of political/religious movements, so if they keep tabs on the radical movements from which terrorists eventually emerge, that could help narrow down the circle of those who are then tracked in a much more intensive and expensive way.
