While this may be a bit of an overzealous act, the police officer may have had a point in doing what he did. If you cannot show proof of insurance, the police officer wants to ensure that should you get into an accident, you have sufficient amounts of insurance to cover the other driver's damages. The first offense of failing to have insurance in your state is pretty steep, from a few hundred dollars in fines to also having several points on your driver's license. The police officers in your state may be able to look up your information but it sounds like he was warning you (so no ticket means no points) and trying to protect you (in case you didn't actually have insurance.