Ignored != dismissed. The NSA has been around since 1952 and has briefly come into the spotlight for collecting data on US persons several times. The vast majority of people of people in the US (AFAICT) don't care. It's not that they don't believe such things happen, it's that they're largely indifferent to it.
This isn't entirely irrational. After all, we both know that the probability of one of us getting blown up by a terrorist is vanishingly small, right? So although we disapprove of terrorism in general, we don't feel all that threatened by it, personally, and think that people ought to have a sense of perspective about the actual probability rather than focusing on how bad it would be if it happened to us. Likewise, the probability of ending up as the innocent victim of government intelligence probe that ruins one's life is negligible for most people. So small, in fact, that it probably cancels out with the risk from terrorism in the mental calculus of the average American.