The FBI is charged with counterterrorism in the United States, and it needs to play that role. Any operations focused against U.S. citizens need to be subject to U.S. law, and the FBI is the best place to apply that law. That the NSA can, in the view of many, do an end-run around congressional oversight, legal due process and domestic laws is an affront to our Constitution and a danger to our society. The NSA's mission should be focused outside the United States—for real, not just for show.
This is a very important point. The NSA has been able to establish a Catch-22 firewall around its behavior due to the nature of its mission.