It's quite legal to refuse cash as payment in the US. You can demand payment in ducks if you so choose, and no one can force you otherwise.

Where folks get confused is that it is a legal requirement for you to accept cash if I'm settling a debt with you. If you were to run up a $50 bill at the gas pump, then go in to pay with your $50 bill they'd be forced to accept it or let you walk. Trying to convince a cop of that may not be the easiest thing in the world though...

Yeap, it's legal: "There is, however, no Federal statute mandating that a private business, a person or an organization must accept currency or coins as for payment for goods and/or services. Private businesses are free to develop their own policies on whether or not to accept cash unless there is a State law which says otherwise."

https://www.treasury.gov/resource-center/faqs/Currency/Pages...

It is legal as long as they tell you upfront. They cannot take your order, prepare it, give it to you, and then force you to use some unexpected method of payment.

Anecdotally: There is a chain chicken sandwich spot in my building in downtown SF that is strictly debit/credit only.