question about booster and laws

[my2kidsSafe]

A friend of mine is a but confused with booster laws. And i can't find a way to explain it. She is EN anyways and plans to keep her son boostered after that until the limit of the seat. But when we were discussing that some states have the 8 or 4' 9" law. The big key in this is the OR. And how boosters are there to allow proper positioning of the seat belt. How is it legally ok for someone over 8 but not quite 4' 9" to not use a booster. Why not put an age with it, and not a height.

[LISmama810]

I would imagine the age minimum is there to make it more palatable to legislators and parents. There's already a lot of backlash anytime a booster law is made more strict, so there is liable to be a ton more backlash if someone tries to pass a law requiring children to be in booster just to a certain height, since you'd have some (gasp!) 12-year-olds in boosters, and that's just a concept that a lot of people aren't ready to grasp yet. I think it's easier to tell people what they have to do with smaller kids than with bigger ones.