They were different times and people didn't know any better. SJWs might as well get mad that those people didn't know about penicillin, airplanes, or cell phones.
Knowing what we know now, those people would have gladly sent the slaves back to where they belong.
“People rented slaves; that is the way it was in those days. People owned slaves. That’s the way it was.”
Didn’t the Lincoln administration use rented slave labor to construct part of the U.S. Capitol during the war to “free the slaves”?
Hopefully someone that knows more about history than I do can comment.