[newtonsmethod]

The issue is that putting fluoride in the water isn't really "treating" the water. It's in essence acting a medication (see my paragraph below for a justification of this), to the benefit of people's teeth. As far as I know, every other chemical added / removed from the water is done for the purpose of the taste of the water, protecting the pipes which serve the water, or disinfecting the water. In this way, it's different from all the other chemicals, and there is also some limited opposition to other chemicals (e.g. debate on the use of UV / chlorine / ozone).

As for a loose argument for why fluoride in water is medicinal: the FDA classifies toothpaste as a cosmetic and also potentially a drug (depending on whether it contains fluoride and the claims the product makes):

> Ingredients that cause a product to be considered a drug because they have a well-known (to the public and industry) therapeutic use. An example is fluoride in toothpaste.

> Some products meet the definitions of both cosmetics and drugs. [...] Among other cosmetic/drug combinations are toothpastes with claims to freshen breath and cleanse the teeth that contain fluoride. [Both quotes are from https://www.fda.gov/cosmetics/cosmetics-laws-regulations/it-...]