I'm not sure how much this is a 9/11 risk?

As far as I know the protection against "taking a jet and crashing it into a building" is a squadron of fighter jets that are quickly dispatched to (if necessary) shoot down any plane on a risky path. That's exactly what happened when some mentally unstable airport employee "stole" a plane and committed suicide by crashing it into the water after a few stunts.

The other risk would be to take passengers hostage, but in case of a chartered plane I think it's fair game to let the customer themselves decide on the level of security they need - presumably they aren't harboring any terrorists/hijackers, and I would think hijacking a private corporate flight is really low on terrorist groups' bucket lists in terms of the expected "payoff".