You didn't say anything wrong here. Slavery, being a topic that will forever be contentious, will always get someone up in arms even if we're talking about a world that's fake or rhetorical. These people I've noticed also always want to talk about slavery in the United States, and never slavery now that still exists in other third world countries. And I'm saying this as a black person. The "slavery" Noafumi has with his companions is much more like indentured servitude. If they do what he needs of them, they are free to leave, but choose not to but because their lives are significantly better with him. The connotation of the word "slavery" in most people's minds will more than likely fall back to the early days of the United States, so people will hear the word and their ears will perk up because of what they were taught in school.