There’s no pathogens in a correctly functioning system.

When something breaks — water pressure drops, or a pipe breaks in a freeze, for example — this can allow random outside stuff into the system, and now who knows what’s in your water, until they can get it sealed off, back up to pressure, and flushed through.

It might be fine. It might be lightly contaminated but good enough unless you are immune compromised or get it in your contacts or up your nose. Or it might be more heavily or dangerously contaminated. So they play it safe.