There really aren't any professional race series where the cars are not race spec. And the cars were never stock, even from the first ever race.
I think people realize the difference between the race car and cars on the lot, they dont buy them because they are the same they buy them because of the name recognition from racing and their teams. People know the difference, they just get endeared to the brand. Without that no manufacture would be in racing. European, American, or Japanese. Its the same in nascar as in all other racing series.