For someone in good health, where the worry is simple bacteria, bringing the water to a good, rolling boil is probably enough.

But the population includes the very young, the very old, and people with existing conditions or compromised immune systems. They're more vulnerable, and a gastrointestinal problem can set off a cascade of events.

A water main break can introduce various organisms -- there are too many unknowns. IIRC, some bacterial and amoebic nasties are harder to kill in the spore/dormant stage.

Given the potential legal implications, they're probably erring on the side of caution.