The reason foridation has additional FDA requirements is because it's essentially a medicine, not a food supplement like iodine - which is an essential nutrient. So to justify the medication of entire communities at the exact same dose of a non-essential pharmacological product in an effort to ensure poor people receive enough flouride seems misguided. Calcium has a similar hardening effect on teeth and is present in water already - forming the scale you see in the shower Ca(CO3). Additionally, flouride stains your teeth [0].

[0]https://en.wikipedia.org/wiki/Dental_fluorosis

Hard water has calcium and soft water doesn't. My city gets naturally soft water and we don't get scale. But we do get added fluoride.

Like toothpaste? You actually think it's easier/cheaper to put it in the entire water supply vs handing out some free toothpaste?

Toothpaste isn't free. Producing toothpaste, putting flouride in it, and distributing it are all harder than putting fluoride in water and distributing it. Water is pumped to people's houses from central locations. Toothpaste has to be placed in tubes and shipped around on boats and trucks.