Given recent historical events in Western Civilization Hitler and the Nazis are the current Bad Guy tm.
Its that simple.
It has nothing to do with democratic elections, cultural emergence or econmic realities of the time period. Most of us were educated through school, or movies, that Nazi's were Bad.
In early times I would suggest that other cultural reference points were used.
Hitler/Nazi references should be allowed only if there is no interpretation or extrapolation made pertaining to the details of the situation. As a reference point to being 'evil', I think they serve pretty well, however inaccurate it is.