Espionage has long been common practice in international affairs, and there is no reason to expect that in the age of technology and information, it would be any different. This rationale is epitomized by the latest revelation that the U.S. is mining both incoming and outgoing data of foreign citizens that communicate in the country. Indeed, the PRISM scandal has brought the issue of international privacy to the fore. Does the U.S. have the right to violate not only its citizens’, but also foreign citizens’ civil rights and liberties?
Legally, the answer to that question is "yes." Reuters explains that the U.S. puts “virtually no restrictions on American spies eavesdropping on the communications of foreigners.” This is to be expected, as information gathering is crucial in helping to protect the U.S. from prospective international threats.
From an international relations perspective, however, the answer to that question should be "no." Both the scale and the form of execution of the PRISM program differentiate it from standard espionage practices that would be acceptable from an IR standpoint. For one, the only criterion that the program uses to give intelligence officials the right to make use of PRISM data is the "foreignness" of a target. Provided that officials are 51% certain that the target is foreign, they are allowed to use any PRISM data they have on them. No other criteria are required. Fundamentally, this allows the U.S. to spy on the world, under the de facto assumption that all foreign citizens are criminals.
Moreover, the massive scale of the program differentiates it from the traditional targeted approach of espionage, which usually focuses on individuals, countries, or institutions, with differing levels of depth depending on the perceived threat of a target. With PRISM, however, the U.S. is in essence firing in all directions. This is not espionage, it is pure paranoia. Granted, the government is not looking at individual data, but (from what we know) it is using an algorithm that searches for suspicious patterns over time. Nevertheless, the data is still there, making the U.S. a kind of Big Brother to the world.
Should this be acceptable from an international-relations standpoint? I think not. While it may be the role of the U.S. government to protect its citizens, there should be limits to the extent to which it can go in doing so. Where there is reason to be worried, espionage may be justifiable. But when you target the world, it is not only paranoia but also an over-extension of the United States' jurisdiction. As a result, although PRISM may be legally permissible, it still stands against IR standards of conduct. Unfortunately, that doesn't mean much, since there are no anti-espionage agreements of the sort that the U.S. could be held accountable against, and espionage is generally not treated as a violation of international law. It is therefore likely that the PRISM effort will continue, despite the international outcry that has been generated against it. To a scary degree, the U.S. is imposing itself more and more as the world's "global cop."