A) Canada is a sovereign country. US laws and $ do not apply.
B) No Canadian business is going to lose money by giving credit for $US.
C) Canadian banks are national, much more used to dealing with things like exchanging other currencies for $CA.
All currencies are traded on the stock markets. I prefer the Bank of Canada, Fed. organization, for daily information on exchange rates
http://www.bankofcanada.ca/rates/exchange/. Note their rates are the rates of major financial institutions with each other. We, who trade in little amounts, say less than $10,000.00 won't get quite such good rates.
:C