>Why can't you just open a U.S. Dollar account for these transactions?

As I mentioned previously, for a Canadian setup, you cannot deposit Diners, Discover and Amex in a Canadian US account. So, as those have to be deposited in Canadian US account, it is the lowest common denominator to have everything in Canadian CDN account. If they would have been like Visa and Mastercard, it wouldn't have represented a problem.