Marriott promises points as a feature of staying at participating Marriott properties as a Marriott Bonvoy member. It's part of what you're paying for, along with use of the room and its features. (A lawyer might use a term such as "implied contract," but I'll just use the word "promise.")
See
https://www.marriott.com/loyalty/earn/hotels.mi
Marriott's sales pitch prominently states: "Check in and stay at any of our 7,000+ hotels across our extraordinary portfolio of brands — including Aloft®, W®, The Ritz-Carlton®, JW Marriott® hotels and more. Earn for more than just your room. Earn on all eligible hotel purchases, including dining, beverages, rounds of golf, spa services and more. You’ll earn 10 points per US$1 spent on eligible hotel charges, except at Element®, Residence Inn® and TownePlace Suites® hotels, where you’ll earn five points per eligible US$1."
You can consider a mandatory resort/destination fee be part of the room rate (because it's part of the mandatory daily cost of staying at the hotel), or you can consider it a "hotel purchase" (because the money is clearly going to hotel, not to a third party). In either case, Marriott is promising points. There is nothing in the Marriott Bonvoy pitch about resort/destination fees being ineligible toward calculating points.
However, buried in the fine print of the huge Terms & Conditions document, "mandatory or automatic charges (e.g., resort charges)" are "Non-Qualifying Charges," so they do not qualify for Marriott Bonvoy points.
That means Marriott's sales pitch is clearly misleading. That's unethical. I would think it's also illegal, although that might vary by country and state.
Reports here on FlyerTalk suggest that members earn points on resort/destination fees at some properties, but not at others. I assume that's based on how a charge is classified in a property's system.
Resort/destination fees are slimy to begin with. Using fine print in the T&Cs to shortchange Marriott Bonvoy members on points is even slimier.