The constitutional standard for all search warrants is probable cause of crime. FISA, however, established a new, different and lesser standard – thus unconstitutional on its face since Congress is bound by, and cannot change, the Constitution – of probable cause of status. The status was that of an agent of a foreign power. So, under FISA, the feds needed to demonstrate to a secret court only that a non-American physically present in the U.S., perhaps under the guise of a student, diplomat or embassy janitor, was really an agent of a foreign power, and the demonstration of that agency alone was sufficient to authorize a search warrant to listen to the agent’s telephone calls or read his mail.
Over time, the requirement of status as a foreign agent was modified to status as a foreign person. This, of course, was an even lesser standard and one rarely rejected by the FISA court. In fact, that court has rarely rejected anything, having granted search warrants in well over 97 percent of applications. This is hardly harmless, as foreign persons in the U.S. are frequently talking to Americans in the U.S. Thus, not only did FISA violate the privacy rights of foreigners (the Fourth Amendment protects “people,” not just Americans); it violated the rights of those with whom they were communicating, American or non-American.
via Government Spying Out of Control by Andrew P. Napolitano.