I can't explain it anymore than I can explain Darth Vader and Stormtrooper costumes outselling Luke Skywalker and R2-D2 at Halloween. Most people like the bad guys.
I'm glad you brought it up. People seem to have no distinction between reality and fantasy anymore.
Fictional bad guys are indeed cool. However, "real" bad guys are not - if you meet Timothy McVeigh or James Holmes, you don't shake their hand - you spit in their f**king evil face. If the same doesn't apply to the Nazis - you portray them not as real evil , but some kind of fictional "bad guys" that were just misunderstood. That in itself is a slap in the face of all those people who perished in Nazi hands or fighting them.
I will not stand silent for that ... EVER!