Subject: Re: Hamas "TERRORISTS!"
As I recall, at least some of the justification was the descendants of Ham. The African peoples were regarded as descendants of Ham, and therefore cursed and "less" than everyone else. I think the "slavery was good for them" is a more recent rationalization (e.g. "if their ancestors weren't enslaved, they wouldn't now live in the USofA!").

But they are correct that slavery is sanctioned in the Abrahamic traditions. In fact, there are even rules regarding how to go about it. Two sets of rules: one for Hebrew persons, and another for non-Hebrews. The Hebrew rules were slightly less harsh, but still pretty bad.