The T&C of the cards state "Reasonable efforts have been made to insure that information regarding participating establishments is accurate; however, AEIS and Sponsor are not responsible for errors or omissions or any decision made by any establishment to fail or refuse to honor the Card." (AEIS is American Express Incentive Services)

While it seems silly for the Monte Carlo not to accept the card, perhaps they do it because there is a higher risk of fraud in the casino industry. However, it seems that they have the right to refuse the cards if they want to.