Technically, it's for obstructing the viewing capabilities to whatever degree that someone might have a blind-spot and run over a kid while backing out of a driveway. Now, that's only one scenario. There are countless other reasons and most of them being about not seeing something.
Another one is.... when an officer is walking up along side of your vehicle, his vision is partially blocked seeing in.... according to what graphics are on the side or back windows. This could prove disastrous to his health if someone inside is carrying a loaded gun and he can't see it through some dumb mambo-jumbo writing on the glass. Same goes for non-factory tinted windows.