Why does Marriott charge a $690 "extra person fee?"

On this episode (Ep. 002), we talk about a hotel fee, even worse than the "resort fee" in my recent case, that Marriott calls the "extra person fee?"
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It's many years later and I'm finding a $25 per person per night extra person fee at the Residence Inn in Isla Verde, PR. With two teens, that's another $250. Could be worse, I guess, since it's not a full-service resort, but man, that really chafes.