Some rich people, especially celebrities or those likely to be tracked by paparazzi, choose to outfit their limo to provide maximum privacy. This involves a few things:
a "limo tint" — An extremely dark tint that only lets 5% of the light through. The goal here is to allow their occupants to be sure their surroundings are safe, while making it all but impossible for outsiders to see in.
Most states have laws that explicitly allow this extremely dark tint, and in some cases, you get the sense that limousines are given special rules that aren't allowed on other vehicles.
curtains or blinds — These again allow occupants to decide when they went to be able to see out, while making it almost impossible for outsiders to see in.