Let's run it down shall we?
Private paid security contractors allowed several young muslim males carrying knives to board commercial aircraft. Those muslim males then slaughtered about 3,000 people using those knives as their primary weapons.
AA hired that security contractor thus bringing into play the doctrine of 'joint and several liability'.
It's really quite simple.
But I suppose it's far better to put the taxpayers on the hook for the negligence of a couple of private companies....
L