That's a good point, but I for one don't necessarily think that it is a given that serving power to all people, no matter where they choose to live, is automatically "the public good."
If the cost of maintaining and delivering service per capita to less dense areas is too high, it might be in the public good increase prices to those areas or stop service.
If the cost of maintaining and delivering service per capita to less dense areas is too high, it might be in the public good increase prices to those areas or stop service.