Marriott International

I made a hotel reservation to spend 2 nights ( 17 July-18 July, 2009) at the Courtyard Hotel using the Marriott Reservation line. I found a certificate at the Marriott website for a “Treat Yourself to a weekend Escape and a free 2nd night”. I confirmed with the reservation person that this was valid for my stay and he did. He said I just needed to print out the webpage and give it to the desk at check-in. At check-in I presented the certificate to the front desk. They called the “Marriott Rewards Hotel Resource Desk” and confirmed it was valid and would be honored. I spent the 2 nights at this hotel and just hours before my check-out I received a phone call from the desk telling me they made a mistake and the certificate was not going to be honored and I was going to be charged for the 2nd night. If I would have been told this at the time I made the reservation, or at check-in time, I would have made other arrangements. I have filed a complaint with the BBB. How can they get away with doing something like this?