Sure, you might not care about seeing out your rearview mirror. And if you want to piss off a lot of people by hitting them when you back out of parking spaces, that's your business.
HOWEVER. If you choose to cover the back window of your truck with a SHINY REFLECTIVE MATERIAL, please be aware that the only thing preventing me from following you and slashing your tires is the very fact that said SHINY REFLECTIVE MATERIAL leaves me effectively blind.
It is difficult to drive a car when your vision consists entirely of white or black. Next time I will punch the gas when this blindness strikes, and hope I knock you off an overpass. If there is justice in this world, your maimed body will be devoured alive by rabid badgers.
Sincerely hoping you crash into a tree and die,
Jenni (and every other person to ever drive behind you while you were heading away from the sun).