So many restaurants in my city are credit/debit only. That should be illegal.
It depends on the kind of restaurant, but cash cannot be refused when paying a debt. https://www.federalreserve.gov/faqs/currency_12772.htm
There is no federal statute mandating that a private business, a person, or an
organization must accept currency or coins as payment for goods or services.
Private businesses are free to develop their own policies on whether to accept
cash unless there is a state law that says otherwise.