Previously all Amex billing was done by Amex separately although they were processed through the one of the Acquirers. The Acquirers often add a per transaction fee on top of the Amex fees. The actual fees from Amex were always very high generally in the 2.9% range and deposits took considerably longer than other credit and debit transactions. Many businesses refused to accept Amex because of the cost and because it was so difficult to balance with the deposits later than others.
Things have changed and pretty all of the Acquirers will add your Canadian Amex account to their billing and the rates for the most part appear to be cheaper. As always though the rates vary and you have to confirm exactly what you will be paying before signing up. Amex customers are loyal and because there are fewer businesses that accept it so it can be an competitive advantage.