Slavery was a way of life for all civilizations then and I assume the Jewish people were very sensitive to that having just come out of slavery. It was how the world worked. Unlike other religions and races though the Hebrews had very specific rules on the treatment of slaves, one being that a slave had to be freed after 7 years. Another being you can't beat or work a slave to death. Man does have free will, if that weren't the case there would be no need for laws biblical or otherwise.