You can avoid skin cancer with a 80% tint too. UV and IR blocking film blocks light you don't see anyway.
Yes, that would provide good skin cancer protection, but then you wouldn't be able to think that you're the cool kid on the block because of your tinted windows and justify it by claiming it's for your health.
I don't think that laws against tinted windows are primarily there for law enforcement benefit. If that were the case, then they'd prohibit tinting of rear windows as well. As someone that frequently rides a bicycle, and to a lesser extent walk on the sidewalk, I can tell you that tinted windows present a very real danger to non-automobile users of the road. Getting hit at even low speed by a car, which would be a minor fender-bender if you are also a car, can easily be fatal if you're on a bicycle or on foot. When on foot or a bike, there are many cases where it's necessary that I see the eyes of a car driver in order to know if it's safe to proceed. That is, do they know I'm there? Until I can establish that that's the case, it can be very dangerous for me to proceed in many situations even if I have the right of way.