Here is part of it, the ride manufacturer and the state DOSH set the height. If there is an accident, and there is no negligence on the part of DIsney, the ride manufacturer bears most of the liability. Whereas, if Disney allows riders who do not meet the requirement and there is an accident, Disney bears the liability. Also, the state spot checks if Disney is training properly for rider height. If the rider is too short, and the CM allows them on anyway, they can be fired, and the state can shut the ride down for employee retraining. Head has to hit the bar, end of story.
My daughter chaperoned a class trip (5th/6th graders) to Great America. She had 6 kids, and 2 who didn't want to ride a ride. She sent the other 4 up the ride entrance. Imagine her horror when the 4 came off the ride, and one girl told her "I was too short, but the girl said I could ride anyway this time." After that, SHE measured the kids before they went up an entrance.
And *I* measured the Jellyfish stick.