To be fair to Marriott, it would put them at a huge disadvantage in aggregation sites where their price (including fees) would be compared with companies that are lying about their price (fees come at checkout).
This is why we need regulation. It’s really hard to be honest when everyone else is lying.
This is what happened with airfare in the States. The reason the government changed the rules recently to force airlines to include the fees for bags is because Southwest, who didn’t charge for bags at the time, operated at a major disadvantage because their prices were displayed alongside other airlines who had these hidden fees tacked on. If you ever wondered why you were not able to find Southwest’s flights on aggregators like Google flights until recently, that is why. Until that regulation went into play southwest prevented their engine to be used in aggregators for this exact reason.
This is why we need regulation. It’s really hard to be honest when everyone else is lying.