This Wall Street Journal editorial explains why.
Some critics have argued that the surveillance now at issue could have been conducted within the confines of FISA. But that doesn’t appear to be true. FISA warrants are similar to criminal warrants in that they require a showing of “probable cause”–cause, that is, to believe the subject is an “agent of a foreign power.” But if the desired object of surveillance is a phone number found on 9/11 mastermind Khalid Sheikh Mohammed’s computer, you may not even know the identity of its owner and you can’t show probable cause…. As for the judiciary, one question that Congressional hearings should explore is whether FISA itself is unconstitutional. That is, whether it already grants the courts too much power over the executive branch’s conduct of foreign policy by illegitimately imposing the “probable cause” standard. Laurence Silberman, a former deputy attorney general, testified on this point while Congress was debating FISA. He also pointed out that while fear of exposure is a strong disincentive to executive abuse of surveillance power, “since judges are not politically responsible, there is no self-correcting mechanism to remedy their abuses of power” in such matters. In other words, FISA grants the judiciary a policy supremacy that the Constitution doesn’t.
