Tesla has been battling with several states for years to get permission to sell its vehicles inside their borders. Michigan has been at the top of the list of states that ban Tesla's direct sales approach, primarily due to its strong union presence and support of the Big Three automakers.
Essentially, states like Michigan will only let Tesla sell its vehicles if the automaker follows the traditional setup. So, it has to sell them through franchised dealerships. However, Tesla sells its own vehicles directly and has no intention to change that.
Each state that bans Tesla sales has different rules. In the case of Michigan, Tesla is allowed to have "gallery" stores and show off its vehicles. However, the cars cannot be taken for test drives and there are no salespeople. If you visit a Tesla store and decide you like the car, you can go online for vehicle pricing and information.