I'm guessing it's to stop fading and stuff inside the truck.
Either that or Merk just has wussy girly-man eyes! =Þ
P.S. In Michigan, you can have any window behind the driver tinted as dark as you want (even painted black) as long as you have two side mirrors. You can have it as dark as you want in the top 4" even with and in front of the driver, but nothing below 4".