My wife got stopped for our tinted windows on the car a while back, and the officer explained that it has to do with the safety of all drivers- being able to see the other driver (and vise-versa). (He did not mention it was illegal because of the safety of officers during traffic stops.)
He made the point of stopping at a four way stop at the same time as someone to your left. Typically people will stare at the other driver, and then make a motion for one or other to proceed. Or if no motion is made, at least you can see the other driver to make sure they are paying attention and they notice you're at the intersection as well- and that it is safe to proceed through the intersection. If the front windows are tinted, this makes it difficult.
With that said, my wife managed to get out of a ticket and we still have the tint on the car.... oops.