the courts upheld warrantless wiretaps. In Brown, a US citizen's conversation was captured by a wiretap authorized by the Attorney General for foreign intelligence purposes. In Butenko, the court held a wiretap valid if the primary purpose was for gathering foreign intelligence information.
It is true that the 5th and 3rd Circuits upheld warrantless wiretaps in these two cases. However, these cases do not decide this issue because a) the primary purpose of the wiretaps must be to gather foreign intelligence, and b) these cases pre-dated FISA.
My view (which is by far the less persuasive and less important of the two arguments) is that as the current NSA program (by the admission of the Administration) is partly intended to prevent terrorist attacks in the US (a laudable goal), one of its main purposes is to prevent the commission of crimes, and hence, the
primary purpose is not to collect foreign intelligence information. Thus, a warrant is required.
The second argument is much more persuasive: FISA supersedes and qualifies this authority, and establishes safeguards and limits to the executive authority.
The counter-argument that FISA cannot supersede inherent presidential authority is lacking:
Youngstown Sheet, 343 US 579 (1952) makes clear that executive authority can be expanded or limited (in short, is defined) by the extent to which it coincides with the will of Congress. As such, by ignoring the criminal prohibitions in FISA, the executive oversteps its lawful authority. This position is fully discussed in a letter to the Congress by a group of law professors (including Lawrence Tribe, Richard Epstein, William Sessions and Deans of Stanford and Yale law schools). Please read the letter as it is much more informative and persuasive than myself.
See http://balkin.blogspot.com/FISA.AUMF.ReplytoDOJ.pdf
Finally, to anyone that argues that since they aren't committing any crimes, so why should we care about domestic eavesdropping, it should be noted that at several times in the nation's history, the government has conducted unlawful surveillance and intimidation of purely law-abiding citizens, the motives of which it disagreed. The most famous of these is Martin Luther King, who the FBI tapped and monitored for years, as well as using the information it obtained to intimidate and discredit him.
See http://en.wikipedia.org/wiki/Martin_luther_king#King_and_the_FBI