It's completely true. If I finish my meal and don't have a credit card, the restaurant will be annoyed. I offer them cash, and they refuse the payment. I therefore leave without paying. The restaurant takes me to court for non-payment and I agree to pay. I pay in cash. From the link in your sibling comment:

"This statute means that all United States money as identified above are a valid and legal offer of payment for debts when tendered to a creditor"