I don't think it's particularly "naive" to believe that retailers or advertising companies could make enough off of a contest promotion to justify the cost of a car. Most of the cost models of contests are opaque and, in most of North America at least, there is probably a general perception that regulations would be in place for something that so prominently implies that it is a contest for a car. The "trick" here is to set up the appearance of a contest while evading the legal controls for an actual contest. The very trick of disambiguating this in the more-or-less fine print is probably only allowed in things that aren't sweepstakes or contests.

Yes, in hindsight, it may seem that only "naive" people could believe this, and that "rational" people see through it. But this strategy probably represents something like the endpoint of a search strategy that found a successful meme, precisely because it mimics authentic, regulated contests.