"why isn't the FBI liable for this sort of thing? Surely there has to be some precedent here one way or the other."

They have no legally enforceable duty to protect...

  ". a government and its agents are under no general duty to 

provide public services, such as police protection, to any particular individual citizen... -- Warren v. District of Columbia, 444 A.2d 1 (D.C. App.181)"