But what's wrong with "Thou shalt not have slaves"?

No. What the bible says is that the Hebrews are the only true men, that other tribes are lower beings and so men of other nations should be taken as slaves, their virgin daughters raped and it's ok to beat a slave to death with a stick.

What has that got to do with free will?

Religion is regarded by the common people as true, by the wise as false, and by rulers as useful.
-- Seneca

http://biblebabble.yuku.com/directory