I agree with you that a USD15 charge is excessive. To share a better way of doing things - there was a similar idea in Australia at the "Sheraton Grand Mirage Resort, Gold Coast". However in that case I felt it was done much more fairly and I was happy to pay the minor surcharge:

1. The water bottle refill stations were all over the hotel (nearest, 2m from my room door), and offered both still and sparkling water - at cold or room temperature, i.e. more choice than purely bottled water
2. The water bottles themselves were custom made and Marriott branded, i.e. something you couldn't get elsewhere
3. The cost was pretty low, believe 5AUD / 4USD, i.e. it felt like it was being offered at near cost, not as a pure money-making scheme
4. As this is a resort most stays would be multi-day, i.e. you could really make a difference rather than saving a single bottle of water

We took the two bottles we bought with us and my wife uses them. They are fairly simple metal (not as elaborated or branded as your example), but keep water fresh.

PS: I see from the "fill it forward website" that the pricing is perhaps not too dissimilar to their own website, so maybe it is legit, just their partner is a bit pricey: https://fillitforward.com/products/cupanion-bottle/