I believe the general rule in the US (unless there are state-specific laws that override this) is that a company is free to not do business with a cash customer, but once they do business with a customer, they cannot refuse cash payment. IE, a store could refuse to sell products for cash, but a restaurant couldn't refuse to accept cash in payment for the meal you've already eaten.

If you ate, and tried to give the restaurant $50 cash to cover the bill, what's the restaurant going to do? Call the cops? The cops will just tell the owner to take the cash or forget about it.