Fluoride is known to be problematic at high concentrations (hell, everything is). The problems of fluoride really start to come into play at concentrations of around 10-20mg/L, and some of the areas being studied are running well in excess of 100mg/L of fluoride.

The EPA limit for fluoride is 4mg/L. There's an argument to be made that it should be lowered to 2mg/L. When fluoride is added to drinking water, the target is around 0.9mg/L--no one's coming close to the EPA limit, and that exists because groundwater sources can end up being naturally high in fluoride. (I'm not sure what the typical natural occurrence of fluoride is in Utah, but I strongly suspect that they're not making any moves to actually remove fluoride from existing systems.)