Ever dreamt of a red fuel-injected 1957 Chevrolet Bel Air convertible with fuzzy dice dangling in your peripheral vision as you cruise on a California highway? That's history, and not just in California. In total, there are nine states that explicitly outlaw hanging anything from your car's rearview mirror: Arizona, California, Colorado, Illinois, Minnesota, New Jersey, New York, Pennsylvania, and Texas. Drivers in Virginia can't hang anything that obstructs a driver's vision, so go nuts with non-obstructing stuff. Florida drivers can hang items such as parking passes, but like Virginia, mirror hangers can't block their view.
Visibility in general is a concern for lawmakers. Several states regulate the height of hood scoops, usually restricting them to 3 or 4 inches above the hood, meaning you can't just have that giant roots supercharger intake rising over your roofline. Carolina Squats are also increasingly becoming illegal because of their deleterious effect on handling and sightlines. Point being, rearview mirror dangler laws exist to help drivers see what's going on around them. Maximizing visibility protects everyone, from pedestrians to motorcyclists, who are already six times as likely to die in a crash as car passengers.