In the letter the scholars provide a lot of background on FISA and relevant Supreme Court decisions, as well as a point-by-point rebuttal of the recent Department of Justice memo (PDF) that attempted to justify the NSA spying program. The letter is well worth reading; here's the conclusion:
In conclusion, the DOJ letter fails to offer a plausible legal defense of the NSA domestic spying program. If the administration felt that FISA was insufficient, the proper course was to seek legislative amendment, as it did with other aspects of FISA in the Patriot Act, and as Congress expressly contemplated when it enacted the wartime wiretap provision in FISA. One of the crucial features of a constitutional democracy is that it is always open to the PresidentÂor anyone elseÂto seek to change the law. But it is also beyond dispute that, in such a democracy, the President cannot simply violate criminal laws behind closed doors because he deems them obsolete or impracticable.