These things are always a tradeoff. The number of terrorists is a tiny fraction of the population, and the amount of damage which they cause is small compared

These things are always a tradeoff. The number of terrorists is a tiny fraction of the population, and the amount of damage which they cause is small compared to road accidents, tsunamis or common and preventable diseases. Is it really worth compromising everyone's communications because of such a low level threat? Collecting everything isn't even remotely proportionate to the scale of the problem, and it's counter-productive even in utilitarian terms. The main issue is that it's not really about terrorists. The NSA recruiters admitted as much in the audio recording in which they attempted to define the term "adversary" to an audience which was smarter than they had bargained for. It's very clear that they consider everyone to be an adversary, whether or not they're engaged in terrorism or crime and regardless of their nationality.