Just because some dealerships have been successful in demanding such an outrageous thing, doesn't mean that it's actually legal.
If you look on the face of a US dollar bill, you'll see the phrase "Legal Tender for All Debts, Public and Private". There's no legal basis for demanding that an asset be purchased with a different type of debt instrument.
Money can't be refused for transaction against a product that has a price on it and is offered for sale.
Anyone who actually experiences a dealership doing this should immediately contact your state's Attorney General (or other consumer protection office) to ask about the legality of this type of transaction abuse in your state. You can look up your particular state's consumer protection agency here:
https://www.usa.gov/state-consumer