Fluoride in drinking water is a kind of truly marginal level of harm that’s perfectly designed to trigger paranoid libertarians.

A tiny amount of fluoride is helpful for bones and especially teeth. Fluoridated toothpaste covers this need perfectly, but many people don’t use it. Fluoride in drinking water covers this need also but (arguably) brings along small risks of symptoms of slight excess fluoride consumption.

The amounts that are actually put in drinking water do not seem to have any significant negative effects, and it seems reasonably clear that the population positive effects dominate, but being non-consensually subjected to a marginal risk of negative effect in order to achieve a positive outcome for the population is exactly the kind of thing that reliably provokes this kind of reaction from the paranoid libertarian set.