A bunch of ancient conspiracies about mind control, and some idiots who don't know there's a difference between a salt with flouride in it and actual flourine (big crossover here with the vaccines have mercury crowd, if you're going to hate vaccines, pick at least a real attempt at an out)
Generally speaking though it does close to nothing for wealthier areas, but most studies claim the salts may have some negative effects but it's not really proven, but it really does help dental health in the poorest areas. Some countries like Japan have ceased fluoridation in richer districts or altogther.
There are some proven issues from it though related to massive volumes of it and fear of it being put into all water and getting too much through crops and livestock. But as far as just running water, it's really likely fine.