What I think happened was AMEX was showing your reservation was a cancelable reservation (rather than a pre-paid nonrefundable). What AMEX doesn't show is the individual properties non cancelation policy. Some properties are 24hrs. It seems this one is 48hrs. I've seen some Hilton resorts even more.

That aside, the booking site is deceiving. I wonder what the link "Cancelation Policy" would reveal. It seems you disputed this with the travel department. What about calling and reporting a fraudulent charge in general?