Some reporting suggested that the NSA and FBI are working together in a way. Perhaps the NSA uses PRISM to spy on foreigners (freely) while the FBI uses PRISM to spy on Americans (with more restraint, hopefully)?
Possibly, although FBI tends to be shackled by a much more restrictive set of laws and guidelines. They tend to get warrants and follow the book because they know that in many cases their work will be eventually made public and scrutinized by skilled defense attorneys as part of a trial. They don't want their cases tossed out because they were based on illegal searches ("fruit of the poisonous tree").
By contrast, if NSA were doing what I have theorized above, they would have unfettered access to all PRISM data with no oversight, and it would be entirely legal.
Has 'fruit of the poisonous tree' ever shut down a line of evidence in a terrorism investigation? Will it? I don't really expect our security apparatus would tolerate a restriction like that. 9/11 changed everything...
If they can't get their way in the courts, who takes the fall for "letting a terrorist go free"?
By contrast, if NSA were doing what I have theorized above, they would have unfettered access to all PRISM data with no oversight, and it would be entirely legal.