

Because a lot of those people are children.
Fluoridating water is cheap. If someone is uneducated enough to not know or care about how their dental health affects their overall health, and the same for their child, it matters significantly less because they get a baseline level of care from the public water system. It means they use less medicare/medicaid funds in the years and decades to follow.
The single study cited in a lot of anti-fluoride laws/debates is talking about possible single digit IQ differences from possible overdoses on the scale of countries. That’s smaller than the effect of the same person taking the same test on different days. It’s a textbook example of finding noise in the data, pointing to it, and saying this proves exactly what I want it to. And it’s being used to remove one of the best documented, lowest cost, most effective large scale public health measures of the 20th century. Because it will not impact people with parents who can afford dental care and/or have the education to value it.
Fluoridation is most effective at ages where it’s hard for kids to physically hold a toothbrush, much less develop good or effective habits. Especially if they’re in a family that is struggling to regularly provide food, much less worry about long term healthcare. Berating children for not knowing things or having self control is a ridiculous position to take.
The entire point of public fluoridation is that it requires no effort or cost by the people it is most effective for. It’s why some places take excessive natural fluoride out of the water and some places put more in because there isn’t any. It’s how we discovered florid was effective in the first place.
Aspirin is poison, Vitamin C is poison, water is poison. “Poison is in the dosage” isn’t a glib statement, it’s how biology works. Putting an amount of fluoride in the water that is still non-toxic if you severely over-hydrate yourself or regularly swallow a tube toothpaste, is not toxic. And it still has massive lifelong positive health benefits that reduce lifetime medical needs.