The NSA surveillance program is designed to address the threat of asymmetrical warfare. An asymmetrical war is one where a strong nation faces a substantially weaker opponent.
Since asymmetric fighters are weak, they resort to ruthless tactics such as terrorism. Recent examples of asymmetric fighters are the Liberation Tigers of Tamil Elam who perfected the use of suicide belts in attacks against Sri Lanka civilians and the Chechen guerrilla fighters who killed 380 people during the siege of Belsan School.
Asymmetric wars last decades and are so brutal that they often give rise to allegations of war crimes on both sides as the response to ruthlessness is equally brutal.
A key concern is what will happen when asymmetric fighters have access to weapons of mass destruction. It was this concern that was at the heart of our recent war in Iraq. While that threat was overestimated, the concern over the asymmetric threat was genuine.
Afghanistan is an example of a rouge nation that combined with terrorists, but luckily Afghanistan was technologically backwards. It's easy to criticize the NSA, but critics have no strategy to counter the asymmetric threat which could even take a global superpower down.