Costco Canada dropped Amex two years ago [1] and I was stuck with an Amex card which is not as widely accepted as Visa or Mastercard. Amex tried/are trying all kind of tricks to not have these cards closed. The best one yet was zero percent for one year - I had colleagues max the card and put that money in their savings account just for the one year interest.
You bet that Walmart negotiated an excellent rate with Amex.
[1] http://www.moneysense.ca/spend/credit-cards/costco-american-...