Water fluoridation rates are well under dangerous levels - about 0.7 mg/L for artificially fluoridated water, vs 2+ mg/L long-term to actually cause any problems. Even that small amount helps with reducing tooth decay, as was shown in the US in studies starting in the 50s comparing areas with naturally fluoridated water with areas without it.

It's significant enough to prevent some tooth decay, but insignificant when it comes to toxicity.

It is a small amount. It helps, but is not enough to completely prevent cavities. You still need to brush your teeth like normal.