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.

For the debt already incurred. No food served, no debt incurred by customer, business don't have to take cash.