It's also illegal if they give you the item before you pay. Cash is "legal for all debts.' So if they let you establish a debt (by, for example, drinking some coffee), they must let you settle it in cash,
"Contrary to common misconception, there is no federal law stating that a private business, a person, or a government organization must accept currency or coins for payment. Private businesses are free to create their own policies on whether they accept cash, unless there is a specific state law which says otherwise. For example, a bus line may prohibit payment of fares in cents or dollar bills. In addition, movie theaters, convenience stores, and gas stations may refuse to accept large denomination currency as a matter of policy or safety."
"The principal purpose of [the Coinage Act of 1965] is to ensure the nationwide acceptance of U.S. currency, consistent with constitutional language that reserves to Congress the power to create a uniform currency that holds the same value throughout the United States. While the statute provides that U.S. money is legal tender that may be accepted for the payment of debts, it does not require acceptance of cash payments, nor does it provide that restrictions cannot be imposed upon the acceptance of cash."
You're missing an important distinction: Cash is legal for all DEBTS. Nothing says the store can't refuse to take cash, as long as they also prevent you from establishing a debt. For example, a restaurant that doesn't want to take cash must force you to pay before you eat.
