Back when I prepped race cars, we would go to Canada for the Mossport race. There's no big towns near the track so exchanging US to Canadian wasn't really possible. The food stands at the track accepted US dollars 1 for 1 for Canadian, which at the time was worth 75 cents, then give change in Canadian. We got hosed both ways.
It's not easy in the states to exchange currency and the vig to do so is high. I don't blame a business for refusing to accept Canadian currency.