I would agree with both of you on this. Yes, it would be unreasonable to expect Amex to clear the fraud concern with really no additional information, but it's unreasonable for Amex to have a fraud concern in the first place. It's a physical card that you're presenting at checkout. These cards are designed to be used anonymously with no way to register them (at least not that I can see), so what could be the concern? That the card has been stolen? That the hotel will accept the card and hand the person a wad of cash? Are these things more likely at those hotels than at other hotels? If someone from the hotel or the customer calls and says the card is not working, what could Amex possibly want to hear in order to say "OK, try it now" ? If a customer has Hilton status, would that help convince Amex that the use of the card is legitimate? Would seeing the final bill help?