Well, because it's not quite that simple.
Snow often needs to be removed during the night or at a 24 hr. rate in heavy storms to keep up with it on main roads. And if all the people are there for overlapping shifts as described, there is not sufficient equipment for everyone to use. Plowing during off-traffic hours is more efficient at times and salting during high traffic can be helpful at other times. It seems that they need somewhat of a regular schedule plus leeway for the bad days and that suggests the need for overtime.