Are you sure it is a city/county vehicle doing it and not a private contractor? I know where I live outside of town some of the smaller roads don't always get plowed and citzens have to do it themselves with their own trucks if they want it done. Maybe it somethng like that. Or the city is just hiring someone on the side to do it. If it is actually an MDOT truck my guess would be they are in compliance with the law, it just isn't something you are used to seeing is all.
As far as I know MDOT doesn't plow roads, they contract that out to the individual counties. As far as I know MDOT contracts all day-to-day road maintenance to the counties.