I'm more surprised that you found a retailer that accepts American Express.
Any major company accepts Amex, its only small businesses and "cheap" places that dont. Amex gives the best cash back I feel for no anual fee cards and therefore sends that cost onto the retailer. So their fees are higher and a lot of merchants wont take it.
But if you are an adult CAG you need at least a Visa and Amex card that gives cash back or some form of rewards. Amex for all the time cash back (Most people have one from Costco) and then the Visa for the retailers than dont take Amex.
My Visa statement is about 10% of my Amex statement. Amex is accepted nearly everywhere now other than small businesses and everywhere online.