Do you have a source for your first statement? Not that I'm implying you are lying. Certainly it's added as a mineral fortifier, but to protect the teeth of people unable to afford toothpaste? That is news to me.
Flouride additions to water was started in the 1960s or 1970s as a public health initiative to fight tooth decay, a significant health problem. It was not added as a "fortifier" in the same way that vitamin D is commonly added to milk.