If you can't afford insurance, the government makes sure you get the health care you need to stay alive and healthy. So the government has a vested interest in making sure people stay healthy, because if a poor person gets sick, the government is paying for their healthcare. Not everyone brushes their teeth, not everyone can afford toothpaste. Everyone has to drink water, though, and fluoride in water causes no real health problems. It's a very quick win for public health, because otherwise the government would be paying to replace poor people's teeth.