You have to ask them a more basic question..., is tap water even SAFE to drink?? At all?
It depends on your location, some are just plain disgusting and yellowish. Some places process it and add chlorine to clean it. But do you think putting chlorine into your body is healthy after a period of time?? Not at all.
You could give them some clear facts. U.S. Health Officials estimate 900,000 people each year become ill - and possibly 900 die - from waterborne disease.
First of all, it's pretty accepted among health affectionados that tap water is not safe. It's rife with all kinds of chemicals to kill the germs - chemicals like chlorine which are not good for human consumption. And since you can't put amounts of poisons to kill germs without rendering the water toxic, lots of germs 'slip through'. One of these, clostridium, a virulent form of one of the friendly bacteria we have in our colons, causes diarrhea and illness regularly in populations who drink tap water.
Secondly, let's take a look for a minute, at the route tap water takes. Through miles of pipes under the streets and homes of the city. Are these pipes a totally closed system? Of course not. Can germs 'seep in'? I would betcha! When it comes to your house, is that a closed sterile system? No way. You have rust, corrosion, seepage and all kinds of things getting into the tap water before you draw it out of the tap.
Third, ground water must be really treated much more aggressively than the city water plants can afford to do, due to world-wide pollution.
Fifth, some studies show that drinking tap water increases the risk for kidney stones. Indeed, when I went to the ER a couple of years ago, and they thought I had a kidney stone, that was one of the questions they asked me, i.e. did I drink tap water.