Because "Marriott" doesn't run the guest's card or take the guest's payment. The hotel with which the guest has booked does those things. Marriott gets paid by the property later on the back end. Pretty basic stuff.

Further, not sure what "adhered to" means in this context - the prepayment terms are for the property's benefit, not the guest's. In most instances, it is in the guest's best interest for the property NOT to adhere to a prepayment requirement.

People who are booking to take advantage of an AMEX offer need to be aware of how the payment system works. In general, your offer won't be triggered until you've completed your stay.