What's stopping some violent billionaire in the Middle East from crashing his own private plane into the U.S.?

I know that it's basically impossible to do nowadays with a commercial airplane because of increased airport security. But what about airplanes that do not go through airport security? There are some airplanes that are privately owned by really rich people and can be flown by them any time they want. And there are tons of billionaires in the Middle East from owning oil and other valuables. I'd bet a sizable portion of them are devout Muslims who do not condemn 9/11. If that's the case, couldn't they hire other people or even do it themselves if they wanted to crash a plane into a building? I honestly don't see anything stopping them, since apparently it's condoned in the Middle East.