Can't book Canada-Asia flight unless you have a Canadian address credit card
UA will change point of sale and adjust the price you pay based on what that is in the new country by credit card billing address. Most airlines don't determine POS this way, but UA does. There are countless number of threads that have this exact same complaint (this may get merged into one of them).
Your choices are to use a Canadian credit card if you have one, or as also mentioned, use a Canadian travel agency (Travelocity.ca or Expedia.ca may let you book these with a US card. Or to do what you did - book with another carrier.