The real reason this movie is derided.
I suspect the fundamental reason that the film is derided is this:
The American Indian is the hero.
Can you think of one single instance in U.S film or T.V where the Indian is shown in his dignity? Can you even think of a single production where an Indian appears where tokenism is not in play? Maybe instances exist, but I can't think of any.
Do you remember any sitcoms showing Indians hangin' out at home, cracking jokes with each other?
The U.S.A is only just beginning to come to terms with their history of slavery. (12 yrs a slave: Directed by and starring black British people - but well recieved in U.S).
The people of the U.S.A haven't yet begun to come to terms with the horrors commited to their native peoples. Seeing the esteemed Mr Depp as a heroic Indian brings shame to them, which they can't accept - so they attack the movie.
(Just my opinion - feel free to respond.)