The hotel wifi service provider business is (and has been for 5+ years) a really crummy race to the bottom. Hotels don't want to do it themselves. They can't really; they don't have the talent in-house. It's fairly expensive to do correctly. Most hotels weren't built with cat-5 installed, so you have to pay someone to go do that. Then you have to install a bunch of networking gear which isn't cheap. Then you have to pay someone to monitor it all and come out and fix it when it goes down. You probably also want some 1-800 number your guests can call when they can't get on-line. The costs add up pretty quickly.
So how do you pay for it all? You're in a hotels.com price war with all your competitors, so you can't just raise room rates. Your customers will get pissed off if you tell them they have to pay extra for wifi. So eventually some genius comes along and gives you this brilliant idea that will make wifi pay for itself, and this is what you get.
See excellent comment from henryl on the hotel business. He's spot on.
Marriott is a franchise business. That is, they don't own the hotels. They license the brand to hotel owners or operating groups. Most hotel brands work this way. Some hotel brands require their owners to use a specific wifi provider or choose from a list of approved providers. Other brands let their owners do whatever they want.
In this case you can see that the owner opted for a presumably low-cost provider that hoped to recoup its costs by displaying ads this way.
Henryl is incorrect in places (places that matter).
Marriott owns hotels, but they don't own every hotel with a Marriott brand on top.
Hotel "brands" can NOT dictate providers AT ALL. To do so runs afoul of anti-trust law. They CAN issue a "brand standard" that you have to have WiFi, and it has to be at least "this good" (insert specification).
Now, where the hotels are OWNED (by any party) the OWNER can dictate whatever the hell she wants.
And Marriott most certainly does own a large percentage of the hotels that sport their brand.