The problem, I believe, is that this is a children's book that presents the true complexity of a historical situation to parents that have no basis in that complexity nor the preparedness to explain it adequately to their children. Slavery = bad is about the nuance you usually get from children's books (if you're lucky - lookin' at you, Berenstein Bears) and if there's complexity, that complexity pretty much has to be the payload of the story... not the background against which the story is set. It's a shame, but an unsurprising one.