I believe as long as the place of business is not a government agency, they are not obligated to accept all payment types.

https://www.federalreserve.gov/faqs/currency_12772.htm

maybe federally, but there are local laws too, including the states of massachusetts, rhode island, and new jersey. Also NYC, Philly, and San Francisco: https://www.usatoday.com/story/news/factcheck/2020/09/16/fac...

Blocking unbanked people from the economy is sorta bad if you have any kind of homeless population.

Not that familiar with the US, but presumably (just because it seems reasonable, and is the case here) they don't have to serve you at all (err, as long as it's not discriminating on grounds of a protected characteristic) so it would be weird for the law to require accepting cash when it also allowed them to nominally comply with that but then refuse to serve anyone who wouldn't pay by card?

cash is legal tender recognized by the country you are in. There should be no rationale to refuse it.

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.