The US dollar is below the Canadian dollar at this time plus Daydreamer is right to say it costs money to exchange the US money into Canadian. Your ATM banking card will likely work in most ATM at major Canadian banks. get Canadian money.
When I go to the US, I use my Canadian card to get US money and pay my stuff in US money. It is not even a question I ask myself. I don't see why it would be different with US residents visiting Canada thinking they can pay anything with US money once here. The person doing business in the visited country does not have to support the burden of exchanging money because we are too lazy to do it ourselves or too cheap to pay the exchange rate.
When I go to the US, I use my Canadian card to get US money and pay my stuff in US money. It is not even a question I ask myself. I don't see why it would be different with US residents visiting Canada thinking they can pay anything with US money once here. The person doing business in the visited country does not have to support the burden of exchanging money because we are too lazy to do it ourselves or too cheap to pay the exchange rate.