It's illogical to simultaneously believe that slavery was a theft of black people's labor and that slavery was economically efficient. You can only hold those two thoughts if you believe theft is efficient.
Worse than that, though, slavery is not merely theft. It is, at a minimum, kidnapping and armed robbery. So "slavery was economically efficient" means "kidnapping and armed robbery are economically efficient."
Now I can see some pedantic utilitarian-oriented economists saying, "well, if the utility gain to the kidnapper/armed robber is greater than the utility loss to the victim..." But among the first to shout them down would be the very same people who are arguing for slavery's efficiency.
I'm still puzzling over why people who sincerely recognise that slavery imposed horrible costs on those people do not incorporate those costs into their estimate of slavery's economic efficiency. Some thoughts:
- They simply misunderstand the idea of economic efficiency, confusing it with profitability or GDP - money measures - not realising that all costs reduce efficiency, whether we account for them or not, and whether we even recognise their existence or not.
- It enables them some perceived moral high ground. It's easy to say you dislike slavery if you think in the way economists do and believe it was economically inefficient. But if it is efficient, then you might like it a little better; you might even think that it's efficiency outweighs its immorality (if, as a cold calculating economist you really do care about moral issues). But we hate it even though it's efficient -- our moral values are not for sale.
- Some of these folks are socialist (and this is therefore limited to that subset), and so they don't believe efficiency comes via the spontaneous order of free markets, but through centralised top-down organisation. Slavery, then, is more efficient than free labor because it is more centrally organised.
- Diminishing the value of slavery takes away the last hope for any sense of nobility for the enslaved. There is some comfort in thinking that despite the evil that was imposed on them, they have an inherent nobility because they are the true source of this great wealth, this great economic growth. If, on the other hand, this moral evil was imposed on them, with nothing good to show for it, no great gains attributable to them, then it devalues their lives even more. It feels nihilistic, ultimately meaningless. It's like claiming that boys drafted to fight in Vietnam died utterly meaningless deaths, when it's so much more comforting to say that our heroes died in a noble cause.
All of these may be part of it. Or possibly they're all wrong, and just the product of an active imagination.