Raising the height restriction might be used as a sort of "shorthand" for a kid's age. By raising the height, they are roughly raising the age of the average rider. It's not a perfect system, but they can't exactly put an age requirement on it, or a courage requirement. I can't believe that they would have opened the attraction not knowing how tall you had to be to get the shoulder harness in the right position. That's the kind of thing accountants ARE good at.