Complete Speculation: I'd guess that the "at participating hotels" text has been there for ages. And, I suspect it will remain there for ages because the people in charge of running that website (MI Fulfillment Services, LLC) probably have no idea how many unused physical gift cards exist in the world.

From a business perspective, Marriott really doesn't need to know. As long as a physical gift card doesn't have value until it's activated, it's not a liability that needs to be tracked.