ssdrvr- Welcome to Autopia!
I don't have an answer to your question, I don't know if it's a pro-only rule or not
But what I *do* know is that enforcement of such laws is, uhm...pretty discretionary; you're at the mercy of the LEO. It's just like front license plates- some people get cited and others never do.
IMO, 50% oughta be clearly OK so unless the LEO is looking to hassle you *I* wouldn't worry about it. Maybe see if the seller of the film offers a "% transmission" sticker.
If you get pulled over, remember to put down your windows, at least the front two, even if it's raining/snowing/whatever. The thing that gets LEOs revved up about window tint is that they don't know if you're gonna shoot 'em when they approach the car because they can't see what's going on inside your vehicle. Yeah, that's a hassle and yeah it's unfair but the LEO is concerned for his/her safety.
You could always call the local BMV office or the local Highway Patrol outpost, but I wouldn't be surprised if they gave you a negative answer whether it's true or not.