The only legal tender currency for transactions in Canada is the Canadian dollar. The hotel cannot give you $US 10 because US dollars are not a currency allowed to do business in Canada.
A modified request maybe why not give an equivalent of $US 10 in Canadian currency for the Food and Beverage Amenity. The answer would be because the currency goes up and down all the time. Right now it is about $US 10 = $CAD 13. In periods of 2007, $USD 10 = $CAD 10. In periods of 2010, $USD 10 = $CAD 8.5. The OP is disingenuous and secretly wanted to say "if the US dollar is weaker than Canadian dollar, give me $10 Canadian. If the Canadian dollar is weaker than US dollar, give me the $US 10 in Canadian equivalent!"
Marriott should totally ignore the complaint of the OP.
As far as I know, in Washington State, a bottle of 500ml pop is about $USD 1.75 - $2 pre-tax at a convenient store. In BC Canada, a 500ml pop is also about $CAD 1.75 - 2.00. Because the purchase power of the $10 is quite on par with each other, there is no need make adjustments.