They didn't want to know, in all likelihood, because most people's water, given a stringent enough analysis, can be found to contain miniscule and likely harmless amounts of various poisons, toxins, bacteria, etc. Around here, most wells contain small amounts of arsenic because of the rock they are in. The lake I get my water from is very clean, but fish live and poop in it, therefore it contains fishy coliform bacteria. This is unlikely to harm me, but the law sez sterilize it.

Common sense, my boy, common sense. In a flood situation, where all sorts of really nasty water-polluting events might be going on, the sensible person boils his water. It would be impossible to instantly update, for example, when a sewer line might have broken.