I've always assumed it's because the product manufacturers (whatever it is) don't want to have a car manufacturer chasing them saying "we don't want our car associated with your product" because they think it might stereotype the car and its image.
If a particular car is seen in say, an AA advert, it implies that car always breaks down. If a car is seen in an advert for pensions, it implies that car is for old people.
It doesn't apply to vans because nobody can tell one van from another (?!) and they don't have an image to maintain in the same way.
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