Are you sure about that? I often used to tell retailers that they "had" to accept, say, 2 dollar bills or whatever because currency says "legal tender for all debts public or private". I found out some years later that this is quite literal. If I owe you money and I want to pay you in US currency-- that is, I want to use the money to pay off a debt-- you are legally obligated to accept it. However, if, say, you offer to sell me a hamburger, but refuse to take anything but 5$ bills, that is okay. (Unless you sell the hamburgers on credit...)

They're not saying you can't use US dollars, just that you get a bonus for using the local currency.

http://www.ithacahours.com/