And you can construct a "rational" argument for it: the slaves are living under an oppressive government anyway and apparently nobody is going to intervene, so if they're stuck in internment camps, may as well give them something to do with their time. Sure, they don't have freedom, but they weren't going to have it anyway, so it's still rational. Right?

In seriousness - if you don't take into account the economic reliance on slavery that jurisdictions with legal slavery (e.g., the US South in the 1800s) had, and you claim it's solely that irrational slaveholders saw certain people as less than human in the abstract, you've missed a good part of the explanation and risk inaccurately understanding how to prevent it from happening again. People had a strong economic incentive to come up with justifications for their objectively immoral actions.