Because Marriott (not the property) is where any reasonable person (but not necessarily someone who knows the ins/outs of the business) would reasonably go. If you make a reservation with Marriott directly, you reasonably expect that Marriott handles everything related to it. I'd expect that 90% of the traveling public has no idea that today, almost all hotels are franchise/marketing agreements and not much more.

Unless the T&C at point of sale tell otherwise, I'd fully expect that I would return to my point of sale to process any refund.