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.
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.