Its a little more complicated than that actually. You see, most longstanding auto dealership laws around the country prevent manufacturers from selling to consumers if the manufacturer also franchises in that state. This is to prevent a manufacturer from undercutting one of their own dealers. However, the laws did not prevent manufacturers who did not have franchised dealers from running their own dealerships.
The situation in Michigan was considered to be open to interpretation, which would have allowed a way for Tesla to open its own dealerships. The governor and the VP of Auto Dealers of Michigan claim that the law didn't change anything. Which begs the question, why put in language that clarifies that all cars must go through franchised dealerships? They were worried.
By the way, the governor gets lots of donations from auto dealers.