I don't know about pay phones but phone companies are highly regulated. In some cases they had to provide local phone service in areas where it was not profitable for them to provide it. It's possible that in order to get the high value pay phone locations they also had to maintain pay phones in more remote places that were money losers for them. I don't know if that's true or not, but I wouldn't be surprised if it was.

So while you did pay money to use that pay phone out in the middle of nowhere, it's possible that part of the cost of keeping the phone out there was subsidized.