The decision to fluoridate is with the municipality afaik, but there are probably federal guidelines. Some places have never fluoridated, and if you want it you need to use tablets. There are also loads of people on well water where that’s supported by the local geology - it’s super common in my area. Well water may or may not contain naturally-occurring fluoride, and usually not at the level of a municipal source.

But that makes it even more amazing IMO, that a consensus was reached and implemented in a decentralized fashion in most places, to the point where it’s normalized. The only other collective action I can recall, which might surpass it in scope and impressiveness, also backed by scientific insight, was the Montreal Protocol.