The restriction on tinted windows here in the UK is for safety reasons.
Try driving around at night with mirrored sunglasses on and see how many near misses you have, especially with cyclists and pedestrians!
The window tint limits required are primarily for safety reasons. In dull conditions and at night, the driver's ability to see is severely hampered by excessive tints. Not all countries have regulations as strict as the UK. In these countries a number of road deaths have been directly attributed to the driver being unable to see properly through a heavily tinted front windscreen. In addition it is helpful for other road users to be able to see the driver of the vehicle, particularly at junctions. Motorcyclists are also subject to laws restricting tinted helmet visors.
Motor Vehicles first used before 1 April 1985: The windscreen and front side windows must allow at least 70% of light to be transmitted through them. Motor Vehicles first used on or after 1 April 1985: The light transmitted through the windscreen must be at least 75%. The front side windows must allow at least 70% of light to be transmitted through them. If the glass is tinted to a point whereby it lets through less light, then the vehicle does not meet legal requirements.
Not "Big Brother" but common sense!