You can get even more granular than that. CDC maintains a list of water systems and whether or not they add fluoride. CDC My Water System. To give you an idea of how granular that is, there are 78 different water systems in my county alone. For most of my life I assumed we had fluoridated water but apparently only 1/78 of our water systems are. I only checked when we had kids and I needed to know whether or not I needed to give them Fluoride Drops.
In our area, the only water supply WITH Fluoride serves an area with a median HOUSEHOLD income of less than $40k with more than 25% living below the poverty line. For communities like these the fluoride is critical because there will be a lot of children that don’t have access to fluoride supplements, or regular care from a pediatric (or regular) dentist.