I have had several work cars, and they get window tinting all around, but the company that does it puts a noticable lighter film on the front drivers and passengers side windows. The back windows are really dark. In the summer it helps a lot!!
I asked one of the managers and he claims that if they tinted it too dark, and the police stopped you with it, you would get fined (and have to remove the film) and the tinting business could get fined by the state.
(don't know if thats true or not)