The reason tinted windows are illegal in some places is because the tinted windows create a dangerous situation for police officers who pull the person over.
I can understand the reason myself.
Imagine you're a police officer pulling someone over and they have there windows darkly tinted where you cannot see the person or people in the car and you don't know what they could be up too.
The people could be loading a gun and getting it ready to shoot at you or could be waiting there in the car until you come up to there car and have them roll the window down.
The driver or passenger in the vehicle could be sitting there waiting to shoot you as you approach the window.
So basically in short the reason windows being tinted is illegal is due to officer safety.