The standards for tornado warnings are not quite that high--they can include actually existing severe thunderstorms that are capable of producing tornadoes. (Tornado watches pertain to the potential for thunderstorms that could then produce tornadoes.) The lower thresholds are surely due to the power and suddenness of tornadoes compared to other weather events.
In rare cases, the NWS will issue a tornado emergency when a powerful tornado is threatening a populated area.
Someone asked about tornadoes and hilly areas--the 1953 Worcester tornado devastated one of the hillier parts of Massachusetts.
And Dennis asked about Manhattan and tornadoes--there have been no tornadoes in Manhattan since at least 1950, although an F2 tornado hit Staten Island and Brooklyn in 2007 and an F1 tornado hit Queens in 1985.
--tim francis-wright