Upvote:8
(I can't view the map so I'm not sure of exactly which locations you're referring to, but based on several of the locations in Utah...)
If you look at the list of locations, most of them have one thing in common - they are locations where it would not be unexpected for you to spend money on other things if you were to stop there.
Many of the locations in Utah are gas stations (Shell, Maverick, Sinclair, etc), and it would be sensible to presume that many people would take the opportunity to purchase gas at the same time, or at least items from the gas station store.
Several others are RV parks, where offering such services for free (even for people who aren't staying there) would likely result in additional customers for their park.
The whole point of the "no such thing as a free lunch" saying is the concept that there is at least some benefit obtained by the person giving you the free lunch - and that is certainly the case at most (if not all) of these locations!