Well yes, of course it is fiction, that comes with the territory of making a film. If people do not know WW II did not happen the way it is portrayed, that is indeed a problem. However, I do not see how the director is to blame for this. People do have some responsibility IMO to be informed, especially when it is about something as important as WW II. Besides, the negative fallout from this movie can in no way be similar to the fallout of that film from the 20s you described, not in the least because back then there was a whole societal (under)current that merely needed an excuse to discriminate against black people.
EDIT: On second thought, I am not sure if you refer to Inglorious Basterds or Django Unchained. My opinion is the same however on both films.
WIt's true about IB that the history being portrayed in an alternative way does not have serious consequences even if people choose to believe that version. But for me, making such movies is still pretty much pointless.
And Tarantino's only point and purpose with IB was giving it a twist of violence. Like WWII wasn't violent enough. I mean, yeah, if you end up thinking Hitler was killed that way, fine, no big problem for the society, but isn't it disrespectful to change the history that is about millions of people who suffered during those times? I just can't agree with these movies.
History, especially history about wars, is a serious thing, and one that shouldn't be taken lightly. Making a movie changing big facts is for me taking the pain of many people lightly.