The countries that don't fluoridate their water don't have minimum levels of fluoride required. Whereas they have maximum levels of fluoride contamination allowed.
I'm not one of those "fluoridation is a government plot to make everyone pliable" crazies. Just saying that it's far, far more important to BRUSH YOUR TEETH than to have a fluoridated water supply. Just seems to me like water fluoridation is just one of those crazy things Americans and places America has influenced do (such as using the customary measure system and circumcising their male children - sure if you dig deep enough you can justify it, but plenty of people get along just fine without it)
The active component of tooth-brushing is removal of mutans-type bacterial colonies from the teeth.
Cavities do not form without cariogenic bacteria, provided with a food supply of simple sugars. If you have such bacteria, there are several ways to inhibit cavity-formation, such as by chemically hardening bioapatite with fluoride ions, or by re-mineralizing tooth enamel.
Communities would be better served by mutans eradication efforts, even though they could cost far more than the $1/person-year that fluoridation costs.
If I had the cash and political backing, I would put mutans-specific bacteriophages and tricalcium phosphate in a municipal water supply. I'd then subsidize dental office visits for persons in the affected area, such that anyone desiring fluoride treatments can get them at no additional charge, and offer gratis chlorhexidine/xylitol mouth rinse after every visit.
It depends on the natural amount of fluorine in the water, if there's sufficient, then it's fine.
It's only in the relatively few areas where there is a deficiency that it's a problem.
And of course it also depends on diet.
Offsetting your claim are studies which show that where people exclusively drink rain water, there is a higher incidence of tooth decay.