Have you actually thought about what is involved in injecting ads into someone else's website? It's actually fairly simple.
When you request a webpage, the injection service retrieves the page, makes a copy, and then adds its own code to the copy. The copy of the webpage (with the extra code) is then sent to the user who requested it.
After the page arrives, it then starts showing ads that the injection service sold to ad agencies. This means that the injection service and the hotel are generating revenue off of content that they do not own or license.
They're republishing everyone else's content with their own ads stuck in. They're making money off of it. Please, tell me how that isn't commercial piracy?
P.S. One source has reported that this type of ad injection is not Marriott corporate policy. I sure hope so, but this service is still based on piracy.