The relationships between businesses and consumers are often unfair: businesses have money and power while consumers have... a few dollars to rub together at most. You aren't under any legal obligation to pay for a no show but Marriott (and the hotel) will leverage their power to ensure you do pay. If, for example, you no show, and you have cancelled the credit card that you had used to secure the booking, and you refuse to pay, Marriott cannot sue you[1] or put you on trial in the town square, there is no legal recourse available to them, Marriott cannot make you pay. But, like any good business, they don't rely on legal recourse, because you value your relationship with Marriott. You can refuse to pay, and Marriott will terminate your account. Marriott knows that you value your Marriott account too much to let that happen and so you pay for your no shows.

Very, very few businesses rely on legal and contractual obligations in their relationships with consumers. Even in industries where they could, like finance, it's often such an expensive headache that businesses will choose to just write off debt. The entire field of debt collection is, essentially, a product of that: sure, a bank could take legal action against a debtor but in almost every case, it's more cost effective to sell the debt on for pennies on the dollar. Marriott couldn't care one iota about their legal options to force a customer to pay for a no show.

Unless there are consumer protection laws that specifically cover a situation and provide consumers with easy to access recourse, you're almost entirely at the mercy of whatever the business decides. If Marriott want to charge you for no shows, or cancel your booking when you arrive because you're not wearing nice enough clothes, they can, because you (and I) accept it, we continue to patronise. If you don't like Marriott charging for no shows, all you can do is patronise another hotel.

[1] Technically, in some jurisdictions, you can file a lawsuit for almost any reason, so Marriott could try to sue you if they wanted to, but they're not going to because they wouldn't win, so for all intents and purposes, they cannot.