Its also from who the business owners perceive as their customers. They probably associate car ownership with higher income levels and would prefer to make their business as accommodating for that customer as possible vs someone who takes the bus and is perhaps perceived as having less disposable income to spend at the business. Sometimes businesses reserve their parking for the most high income customers e.g. when they make it valet with a high price vs first come first serve.
Is this research comparing spending at the same store, or just car owners vs non-car owners buying every day things? If the latter, I would think it's more that if you don't drive, you likely live in a more expensive area and things cost more.
