There's a free refill policy at Starbucks too, but it specifically states the constraints, which I used the hell out of for years, and is just part of the deal. It's advantageous in a number of different ways for them, but if it wasn't they can take that out of the contract. Worth noting that it still remains despite lackluster and varying performance in the market; their milk sales would compensate anyway.

"Free refill(s) of hot or iced brewed coffee and tea. Starbucks Rewards members may receive free refills of hot or iced brewed coffee or tea during the same day in store visit at participating Starbucks stores (excludes Cold Brew and Nitro Cold Brew, Iced Tea Lemonade, Flavored Iced Tea, and Starbucks Refreshers® base). To be eligible for free refill(s) of hot or iced brewed coffee or tea, your initial order must be served in for-here ware or a clean reusable cup."