If the rural town of 500 can't support an FD, perhaps they shouldn't be an ongoing, existing town.
The same logic applies to roads, medical, internet service, food service, carpenters, plumbers, etc.... No one asks the workers in those fields to work for free.
As for towns banding together, they can CERTAINLY band together for training.
And the other solution is to have a regional company that can't get to many fires quickly enough to save the first location, but in time to stop spreading. The region would have to accept the risk of losing a few buildings each year, and they'd pay anyway due to higher insurance costs, and lower resale value. It's just people asking for a free ride that can't go on forever.