Fluoride helps to protect or repair tooth enamel, granted they are not overused. This is the same principle of putting minute amounts of fluoride in water, mouth washes or toothpastes to aid dental health. There are lot of conspiracy theories surrounding fluoride but is completely safe when administered at proper dosages (too much of anything causes problems)
Here’s an excerpt from web MD on fluoride and its use:
Fluoride is a mineral that occurs naturally in many foods and water. Every day, minerals are added to and lost from a tooth’s enamel layer through two processes, demineralization and remineralization. Minerals are lost (demineralization) from a tooth’s enamel layer when acids — formed from plaque bacteria and sugars in the mouth — attack the enamel. Minerals such as fluoride, calcium, and phosphate are redeposited (remineralization) to the enamel layer from the foods and waters consumed. Too much demineralization without enough remineralization to repair the enamel layer leads to tooth decay.
Fluoride helps prevent tooth decay by making the tooth more resistant to acid attacks from plaque bacteria and sugars in the mouth. It also reverses early decay. In children under 6 years of age, fluoride becomes incorporated into the development of permanent teeth, making it difficult for acids to demineralize the teeth. Fluoride also helps speed remineralization as well as disrupts acid production in already erupted teeth of both children and adults.
It is certainly important for infants and children between the ages of 6 months and 16 years to be exposed to fluoride. This is the timeframe during which the primary and permanent teeth come in. However, adults benefit from fluoride, too. New research indicates that topical fluoride — from toothpastes, mouth rinses, and fluoride treatments — are as important in fighting tooth decay as in strengthening developing teeth.