The book is starkly non-political, appropriate for a children’s book, and the fact that it remains so even when discussing an issue as divisive as the Civil War is unacceptable to many on the left.
A children’s book about a horse that served during the Civil War cannot be just a children’s book about a horse: it must make a moral judgement on the Civil War and the Confederacy to be acceptable to the left. https://www.campusreform.org/?ID=10920