The missing ingredient in the US is a lack of public health infrastructure that universally covers poor people. Obama's healthcare reform didn't even cover every poor person in the country. But if we had that, adding in a fluoridation regime would be trivial. "Fluoridating tapwater is the cheapest way to get it to poor people" is only true because so many poor people in the US have no healthcare, period, so you have to set up all the infrastructure from scratch. Dumping it in the city water is cheaper than setting up real public health infra, but only before you factor in every other benefit of having public health infrastructure and all the cost savings across all of society caused by having public health infrastructure.
Neoliberals in the US love it because it's one of those "smart" solutions that requires absolutely no national-level infrastructure solutions, you just need companies with fluoride waste on one side, and municipalities willing to buy some on the other. You don't have to make our society better, and what's more, you can castigate opponents for hating poor people when really you're the one preferring dumping a single chemical in the water to address a single type of dental problem instead of supporting actual public dental health infra in this country.
Where does "no, people don't have the right to choose if [chemical] is added to their bloodstream, because they are stupid" stop? Who determines when it's "stupid" not to add a chemical to the water supply, and to whom do they answer? If the voting public decides to override public officials on a matter like this, you're basically saying they shouldn't have the "right" to vote the officials out on those grounds. Because this is a very ordinary policy matter, and it's subject to the exact same mechanisms of democratic review that exist for other policies in this society.
If you're a Republican, well, they're inconsistent, evil psychos, I don't expect much from them to make sense. But if you're a Democrat.... if you're a democrat.........