USD AMEX conversion SCAM

Several weeks ago I booked a trip for my family, we didn’t want to travel to the US like we normally do because of high conversion rates so we booked a “local vacation” on airbnb.  Our account price is set to CAD and language to ENGLISH, we have a Canadian billing address and a CANADIAN issued American Express which we used to book.  We booked a Canadian host. we just got our card bill and got a 200+$ conversion charge to USD (from $640 to $867!!) When we look at our AirBnb receipt and even in our transaction records it says it was 640 and our account profile is definitely set to CAD and the hosts currency is definitely set to CAD.  I chatted with customer service and they said it was because we use an American Express card and it HAS to be charged is USD.  This is ridiculous and I phoned American Express Canada and they said that makes no sense whatsoever as my card is a Canadian issued card and they have moved forward with disputing the charge and are going to investigate. I just can’t believe how ridiculous this is. I was a big fan of airbnb’s business and model and now I’m treated like an idiot because I foolishly assumed that if I booked a Canadian room on the Canadian site with my Canadian Billing address and my Canadian AMEX card with my profile setting set to CAD dollars and language set to English that I would be charged in Canadian dollars! C’mon air bnb!