We provide a service by providing them with the information on the 3rd party rates -- so they can eliminate the "problem" and it costs them a discount for 1 person.

If you have ever seen Miracle on 34th Street, the same thing works. Macy's will know and refer a customer to other stores when they don't have something. That way, you go to Macy's first. In this case, you go to marriott.com first and stop there. (except the FT crowd who will look for a lower rate to win the discount.) Just like the stores that offer the 110% price match or Building 19 which price matches and gives the customer a bottle of "Champaign el Cheapo."