I distinctly remember thinking in my high school Gov’t class that the exclusionary rule was weird. Basically, the idea is that the primary mechanism for enforcing the 4th Amendment’s protection against unjustified searches is that evidence collected in violation of this Amendment cannot be used to convict someone. But this is weird because (a) it can lead to setting free people guilty of egregious crimes because of minute privacy violations and (b) it offers zero protection against privacy violations by the government for other purposes, such as convicting third parties. I always thought it was kind of missing the point.
(There doesn’t seem to be a good pure check againt privacy violations to be found in the court system. Right now, you can apparently sue the federal government through the Federal Tort Claims Act for privacy violations, but only if the government agrees. Similar situations exist with the states.)
Now, as it turns out, problem (b) is front-and-center in the debate over FISC‘s powers. It’s true that normal criminal courts grant warrants in a non-adversarial setting, just like FISC does. But tptacek and dragonwriter point out on HackerNews that this is defensible because there is an adversary when this warrant is actually executed, and exclusionary rule can be used to rebuff unjustified warrants.
On the other hand, there is no defendant to challenge the warrant in the case of mass surveillance of the public. Anyone charged with with a crime as a result of this surveillance cannot claim the exclusionary rule, and people whose privacy was violated cannot (almost assuredly) get compensation. This second effect is even more true when the government uses the information, not to convict anyone of a crime, but to pursue extra-national goals like hunting terrorists. PRISM, in a sense, is the largest possible exploitation of the third-party hole in the exclusionary rule.