I suppose this is a valid point.
I'm not sure I'd let someone test drive a new car if they told me they were only doing it because they really, really wanted/needed a $15 gift card, but...
yeah.
In Atlanta many years ago, we got something like 40,000 Delta miles for test driving a Buick. When two of us got to the dealership, we were met by what looked like a high school kid. He asked if we really wanted to test drive or were just there for the miles. We told him, MILES! He signed the form and we were done. Seems that over the previous days of the deal, people were wasting lots of salesman-hours for testdrives. The salesmen were not amused and I guess were not consulted on the whole deal.
Once I got a $50 Amex card for test driving a vehicle with OnStar in it. The dealer didn't know about the OnStar thing but only had to sign a form. The only vehicle they had with Onstar that qualified was in the back of the lot and blocked in by many cars. We got in, pushed the button and after many minutes got the recording. I asked for directions to a nearby location and the call dropped. The salesman shrugged, signed my form and I left. He was hoping to see how OnStar worked too as he had not tested it yet (he was a new salesman).