If sodium floride is a hazardous waste bi-product- why would they put in our drinking water?

Small amounts good. Like many things (vitamin c, iron, oxygen, water), Fluoride in the correct small quantity is safe. It is only toxic when systemic exposure is much higher then the recommended amount. At the correct amount Fluoride makes your tooth enamel more acid resistant and lowers tooth decay. That is why small amounts are in drinking water. In reality you need a lot over a long time to have any problems.
Concentration. It is a matter of dilution. A little bit is good, for bones and teeth. Too much is harmful.