It's for safety! Yes that's the world we live in. The police man touches your taillight when pulling you over so as to leave their finger print behind just in case something happens. Pure genius but sad to learn that's the reason. Have you noticed lately that when you see someone pulled over there are now 2 cops and not just one? I may be the only person who realized that but I swear I haven't seen just one cop pulling someone over in forever!
Read more about the other reasons this happens from Tip Hero.