I find that the decision on which to use is often done by people who are trained to make those decisions. In most cases they know what is in the raw water from the source and some of those items they need to study are what they start with, what their piping, pumps and storage involve as well as the expense. Once the supplier goes to the tremendous expense of adding chloramine equipment, they normal continue to use it. But that does not mean they don't also use chlorine when it is better suited. Chlorine burnouts are one that we may hear about as those are cases where the supplier may post notices before the burn.
How water treatment is handled is not a federal decision to be fought from a state's right positon. There are federal regulations, passed to states, passed on down to local groups. In this process, there is a good deal of flexibility for adapting the rules to fit the local situation. And at times, as we've seen in the Flint, Michigan case, the decisions are made for the wrong reasons and people suffer.
When politics, religion, or economics become more important that scientific and medical information, we may all suffer. When this posting changed from a posting about useful information and became a rant that is not based in science, we also lost an opportunity to learn more about our water.
I'm done with this waste of time.