>if the NSA had that data, they'd share it with the FBI
No they would not. Different government agencies are tasked with different resources, different objectives, and different guiding laws. What is legal for some agencies is blatantly illegal for others. The NSA has no domestic mission and cannot generally operate on American soil - true of the entire US military [1]. This blurring of those lines is exactly the problem. By permitting the FBI to take on NSA-like capabilities, and giving the NSA FBI-like responsibilities one gives the military a role in monitoring its own citizens, and an adversarial law enforcement agency unlimited espionage powers - very dangerous precedents - digital or otherwise.