Think It Through
The Foreign Intelligence Surveillance Act puts the most minimal restrictions imaginable on getting a intercept warrant, so why ignore the requirement? Some have opined that they don't want anyone to know who is being monitored, but that's just silly because the court is a special creature that has never shown itself to be "talkative".
The other question is: why use the National Security Agency for the surveillance? NSA has a lot of capabilities but telephone tapping is not exactly its "cup of tea". This is "trowel" work and NSA has bulldozers.
Off the top of my head I can think of a very good reason for avoiding the court and using NSA for the intercepts: this isn't a targeted operation, it is a hoover
They have resurrected a variation on John Poindexter's Total Information Awareness
program and are data mining international telephone calls and e-mails. They have no names to put on the warrants, because they aren't looking at individuals, they are looking at classes of people. They were told to shut down this program, but they just change names and offices and continue their illegal activities.1. [from the vacuum cleaner brand name] Sucking up whatever is available and seeing what filters out.