What is with refusing to admit slavery is wrong?

[CapnAwesome]

Why do religionists refuse to condemn slavery? I hate arguments like "it was normal back then", "everyone else was doing it" or "so-and-so put rules in place to make sure slaves were well treated". Why can't they just admit the author(s) of their religious text(s) were influenced by moral relativism? Why is it so hard for so many people to say that slavery is wrong?

I think this coin can be flipped a couple of ways. Yeah it's crazy how religious people who are moral absolutists suddenly become moral relativists when you point out the terrible morals of either their texts or their founders. Muslims (duh!) are the worst about it. Maybe because their founded was so obviously immoral.

I'm an Atheist but I also believe that morality isn't relative. Slavery isn't wrong because everybody says it's wrong. It's wrong because it causes so much pain and suffering.