I think the demo cars are unregistered manufacturer vehicles, just as a new car would be. In fact I believe if someone buys a Tesla, registers it, uses it for a day, and give sit back to Tesla for whatever reason, it is then technically a used car as it has been registered to someone before. This would also make the car ineligible for the $7,500 rebate.
That is typically how it works. Here in Michigan though Tesla can't sell cars and can't get dealer plates. So they might have had to purchase and register them to the company to get test drive vehicles.
If anyone happens to go by Somerset, take a look please.