My guess is the presence of the fighter jets gives the people on the ground some control of the situation. If there is a terrorist on board and he forces the plane to head somewhere other than the planned landing point, then the people on the ground can give the order to shoot the passenger jet down. Of course, this would be the decision of last resort. The passengers would be considered unavoidable casualties in the attempt to avoid greater casualties.

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BS in Accounting(General) from Excelsior College
Enrolled in MBA program at Upper Iowa University.