If America had laws regarding the treatment of slaves, and the responsibilities of the slave to their master... that implicitly approves of the institution of slavery.
I wouldn't suggest the bible promotes slavery, but it doesn't outlaw it thus slavery continued under Christians. Now today, we view slavery as something very abhorrent under any circumstance, but I don't think we should as theists fall into the ever-changing-secular rules of morality. It shifts rapidly, things which were immoral yesterday are moral today, it makes the very idea of morality arbitrary.
I don't support slavery, but under certain circumstances it's very understandable. What if someone who had no money had to pay off a debt, and worked as a "slave" until that debt was paid?
Unfortunately as time went on slavery was made an evil thing, rather than seeing a slave as seeing someone who owed a debt to society or a person, they were not seen as humans at all, treated like animals, not allowed even basic necessities except for what would keep them alive (and often times not enough to keep them alive), were not allowed education... that doesn't help society at all.