It's a very bizarre premise for a story IMO. It sounds like it was a book meant for pretty young kids, and I don't think they need to learn about slavery at that age. That will come later on in school.
I also agree that it's odd to make slaves seem so happy and nonchalant. While it's true many masters were actually not brutal in the treatment of their slaves and many slaves had an okay life (as good as it could be in the situation), I think it sends an odd message to the kids. Just a weird book overall.
I think it's good it was pulled. It's not even about being offended, it's about reading the description and thinking... wtf. At least for me.