If you want to steel man the argument you should point out that the maximum allowed fluoride levels in US are quite a bit higher than in, for example the EU (on the order of 3 times higher), and that some recent studies have indicated some potential health risks for young children who consume a lot of water around the very top end of what the US allows.

Of course the correct response to this is to overhaul the recommendations and lower the maximum allowed levels, not to issue state wide bans.