> if we suddenly discovered that cows were sentient

People eat beef mostly because they’re used to it and they think it’s good for them. Everybody knows cow are sentient, there’s a strong intuition (why wouldn’t they like other animals ?) but also tons of literature. There’s not much doubt about it neither.

I agree with the slave owners, however the spectrum of acceptance is large where it’s part of the society. What about someone that make profit by doing business with the slave owner? Someone that buy products coming ~probably~ from that work?

Or someone assisting an "indigene showcase" because they know nothing about this humanoid that look, speak and act differently than the people they used to known (that are from 100km away max). Not different than a zoo, and both are tremendously cruel.