An old friend of mine, a mathematician at an elite college, told me some time back that—while still a student—he or she had done some work for NSA and been greatly relieved by the strict assurance given in the personnel orientation that the agency does not spy on Americans. Starting with the revelations in 2005 about the warrantless wiretapping program, this person recounts feeling a bit undercut, betrayed even—as though the services had been procured under false political pretenses.
This conversation has been very much on my mind recently, because it points to a problem that, I have come to believe, lies at the true core of the NSA disputes: The law is so dense and so complicated that it cannot be accurately summarized at a level a citizen can reasonably process.
Any effort to summarize the relevant law necessarily ignores themes sufficiently important to its architecture that the reductionism will partake of serious inaccuracy. The person who told my friend that NSA does not spy on Americans was not lying. He or she was highlighting a crucially-important limitation on NSA's authority vis a vis US persons. The law and the relevant regulations all contain significant territorial restrictions and significant protections for US persons overseas as well—all designed to separate the foreign intelligence mission of NSA from both domestic intelligence and domestic law enforcement. It's a sincere and pervasive effort. "We don't spy on Americans" is a common shorthand for a wealth of law and practice that really and meaningfully keeps the agency out of the business of being a covert domestic intelligence agency.