In proving Moore's Law, all the world's information can be stored, catalogued and accessed in ways that would make J. Edgar Hoover leap with delight.
Meanwhile, the old system of separating domestic and international spying is looking more and more antiquated. It's fine to have a 1970s-era principle that the FBI can't spy on radical citizen activists unless it has evidence that they are looking commit a crime, while the CIA can spy on the Soviets with few limits. But in a world where loose networks of terrorists incite our biggest fears and where information is pinged around the world with little respect for national boundaries, the old distinctions aren't particularly useful.
What could or should the intelligence agencies have done with Tamerlan Tsarnaev, the deceased Boston Marathon bomber? He was a legal resident of the United States who was not known to have committed any crimes before the attack but may have had connections to Chechen radicals. These situations rapidly become a legal knot, and it's hard to know from constitutional principles exactly where the lines ought to be.
Couple that with the extreme secrecy around what exactly the U.S. government's technical capacities are, and what legal authorities they are based upon, and you have a nasty combination. The people (up to and including the president) who know what Prism and similar programs are truly capable of argue that disclosing those details would make it too easy for bad guys to evade government monitoring. So we have to just trust that they aren't overstepping any boundaries of legitimate civil liberties.
So, it's: Just trust us on this.
Put it all together, and we end up here: Technology has made possible the monitoring of private communications on a scale that was unthinkable a generation ago. The legal regime that governs use of that communications has evolved slowly to deal with that reality. And the people who actually know what the government is doing, and what it's capable of doing, say they can't tell us those details. Is it any mystery why people are uncomfortable with what they've been learning in the last few days?
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