Patent Law has some interesting points. I suppose a lot of it has to do with payment and delivery. At the time the client pays for the vehicle, or signs the contract, the attendees to the car are responsible to him. They are only holding the car he owns till he picks it up, or has it delivered. In that period, anything unusual which happens should be disclosed to the owner. Granted, he has the right to inspect the car, again, at delivery and call the deal off if there is anything odd, like excess mileage or damage. However, most people assume the car as delivered is the car as seen on the showroom floor. Once the buyer accepts delivery, any claims become very difficult to support.
On the other hand, a car on the dealer lot is open game. Anyone at the dealership could be doing whatever they want with it, realistically. Think about the abuse a half a dozen thrill-seeking test drivers would cause.
The difference is this car was bought and payed for, and the buyer was only expecting delivery. This should not have been an "unknown commodity" still subject to the whims of just anyone eager for a test drive.
Now I've seen cars on the dealer lot that were "sold" and I have still been allowed to test drive them. So there's some leeway here. I don't think there is a reasonable expectation, however, that the transporter would have the same kind of access to the car. Bottom line is, if you buy a car--take it home. Don't leave it sitting around or have thrid parties pick it up, hold it, drop it off, etc. You are only asking for trouble.