MacArthur doesn't say slavery was a "standard" human situation, but a "perfect" one.

You changed the language.

When citing the Bible's rules on slavery you refer only to slavery of Israelites. The slavery of Canaanites had no restrictions on abuses (Lev 25:45-46).

As MacArthur came in view of prominence as an Evangelical cleric in the 1980s, the person he'd been before seems to have been largely overwritten. Perhaps he's so ham-handed with the racist talk because it's so new to him.