As this concerns air traffic controllers in the US who are employed by the FAA (but of course there are many other flavors of ATC employ out there)... The Federal Tort Claims Act imposes liability on the US government for the wrongful act or omission "of any employee of the government while acting within the scope of his office or employment, under circumstances where the United States, if a private person, would be liable to the claimant in accordance with the law of the place where the act or omission occurred. . . ." In other words (and speaking very generally), so long as the act or omission was "within the scope of employment," the US government takes the tort hit. If, however, the controller goes rouge and starts acting outside the scope of his or her employment, all bets are off.