There is one very important question about government surveillance that I have not yet seen discussed: What happens to the data?
To see why that matters, imagine it is 2016 or 2020 and the candidate of the incumbent party faces a serious risk of losing. Someone in the security apparatus, loyal to the candidate or believing that the election of the opposition candidate poses a serious risk to security, starts looking through a massive database containing records of all phone calls made over the previous ten years—who called whom from where—looking for calls made by or to the candidate. He finds in the pattern evidence of an extra-marital affair. He waits until the candidate has been nominated, then leaks the information to a friendly reporter. Or imagine that some important bill is up in Congress and the vote is very close. A senator opposed to the bill gets a call from someone who makes it clear that he has somehow obtained information of misdeeds by the senator, political, marital, or legal, and the information will become public if the senator shows up and votes against.
Modern technology makes it possible to inexpensively store and access vast amounts of data. The hard drives I own have a total capacity of several terabytes; a terabyte is enough to store a significant amount of data—about six hundred words worth—on every person in the U.S. The much larger storage facilities available to the National Security Agency should have no difficulty holding the complete calling records of the entire population over a period of decades. And once the information is there, whatever the legal purpose for which it was collected, it can be used for other purposes.
Whatever else comes out of the current controversies, one thing that should come out, and probably will not, is a requirement that all data collected and not used be erased within a reasonable period, say a year, after collection.