The key words printed in EVERY hotel booking confirmation for Marriott: Please note that a change in the length or dates of your reservation may result in a rate change. They're on the website during the booking process and on the confirmation email.
You may have a leg to stand on if during check-in you told them up front you were staying one less day and the FDA affirmatively told you there would be no rate change. But good luck proving that - it's the FDA's word against yours.
-RM