Because, while he will now wear a seatbelt, he can foresee times that his majesty may need to be delivered somewhere at such speeds again.
Since his driver is a state trooper, and it's an official state vehicle, that would be lawful IF the vehicle is equipped with grill lights and a siren, in the manner of an unmarked police car.
I suspect it's not the devil that kept this idiot alive, but Someone with a Higher Purpose in mind, perhaps just to make these commercials and maybe save a child who is to become a future President, great US Military leader, or something like that.