They can use UV light, ozone, or even chlorine, but they remove the chlorine before pumping the water into the system.
The downside is that it requires a highly precise tuning and maintenance of the water systems to avoid any contamination downstream. Most countries don't seem to have the discipline to do so (US for example).
But they do fluoridate the water, depending on the country and region.
Water fluoridation's positive impact is small when the population frequently brushes their teeth with fluoridated toothpaste, and both fluoridation and chlorination don't have any negative impacts on health.