After all, it's a chemical cousin to chlorine. Fluoride is pretty nice stuff. Unlike chlorine, it doesn't react to produce anything nasty. And it's not a carcinogen itself. The traces of fluoride we put into water really do drastically cut tooth decay.
Chlorine is another matter.
It tells the tale of our technology-dense world in microcosm. We trade a few deaths to avoid many. That's a not a deal we can accept in the long run. Since 1974 we've fought it. We've brought millions of dollars to bear on the problem.