They have the right to use cash, even if the vendor chooses not to accept it.
I learned this by trying to pay a fine with coins, which are NOT legal tender like cash is.
https://en.m.wikipedia.org/wiki/Legal_tender
> Each jurisdiction determines what is legal tender, but essentially it is anything which, when offered ("tendered") in payment of a debt, extinguishes the debt. There is no obligation on the creditor to accept the tendered payment, but the act of tendering the payment in legal tender discharges the debt.