Nitpick mode:
That service doesn't provide free cars; they claim to provide--for a fee--information on programs that either provide you a car or pay you to drive "your" car. Note there aren't any details, like how many people actually can do this or how they are selected.
I am reminded strongly of the enthusiastic guy on cable TV commercials...he wears a suit covered in question marks and hypes his book on how to get money from government programs.
I actually signed up for something similar when I was 18: This service would provide a list of college financial aid for a fee...all I had to do was resell the service to other college applicants. I think they quit business promptly after I paid them.
Anyway, my guess is that the "free car" thing is information marketing studies that provide cars possibly combined with some info on contract jobs that involve transporting rental vehicles to new locations or delivery.
My aunt and uncle got into some marketing sampling a while back. They didn't get cars, but they tested some other stuff and weren't allowed to talk about the product while they had it. They also had to log detailed information about use and impressions.
There's no such thing as a free lunch car.