I think even if it was not law most area's would use a sanitation agent, as a insurance preventative measure (think of the lawsuits). And they would be unlikely to be able to buy insurance without it.
Pipes are constantly breaking, and can be a source of contamination, many breaks can go unnoticed for years even decades
Also the inside of old pipes (any material) usually becomes nasty fast. One look and I think most people would want a sanitation agent as well
Im not sticking up for any state, but I think a sanitation agent is the lesser of 2 evils, but see no reason why those who want it cant treat their own water.