Historical legacy. It made sense when they were first rolling out. Someone would take the risk of trying to build up a market for these really expensive new devices and then the factory would swoop in and undercut them and destroy their business after they had done all the initial leg work of creating demand for the vehicles. They wanted protection from this.

Well, cars are now everywhere in the US market and it doesn't take a whole lot of effort anymore to convince someone they need a car, and not just a horse. But the laws protecting "car market development" in the former of dealerships never went away.