The reason that the tint laws are different in each state is that the responsibility for setting regulations regarding cars (after the manufacturer is done building it and with the exception of smog rules) falls to the states. One of those Constitutional amendment thingies.
As far as tint goes, I think it looks good in moderation and has a functional value, especially in the desert SW. However, the tint rules are in place basically as a police safety issue, so they don't get shot performing a traffic stop. With a couple of police officers in the family, I'm OK with that reasoning. State laws vary from state to state depending on how strong your state's cop lobbying group is and how permissive your state legislature is.
Of course, most cops (especially local ones) really get cheesed just because they can't watch your every move through the tint and that hurts their power ego. You just might have some doughnuts in there that he wants to check for contraband.
