This is because the fluoride interferes with the normal development of teeth. As I understand it, this is because the fluoride replaces some of the calcium used to build healthy teeth. Anyone who is truly interested in the health of their own or their children's teeth would be well-advised to remember what their mothers told them. Avoid sugar, keep your teeth clean, eat a nutritious diet. Chlorine kills organisms that could make people ill.
Water purification is essential - however, there are alternatives to chlorine.
Fluoride is not a necessary water additive. It isn't even an essential nutrient. It's only added to water supplies for political reasons with the claim it reduces tooth decay in those who drink it. And the fluoride found naturally is usually calcium fluoride. The drinking water additive is contaminated industrial waste products.