Depends on the campaign, really. In the current game I'm in, slavery is the common punishment for defaulting on a large debt. Incur a debt, then don't pay it off, then taken in for judgement - if your assets, in liquidated, would be insufficient to repay your creditor you are given to him as a slave where he can have you work to repay what you owe, sell you off to recoup his loss, etc. Prisoners of war left unrecovered by their home country (or who arn't willing to return for whatever reason) also usually end up on the auction block.
However, the difference is that in most countries in this game that allow slavery (all but one, really), the owner is required to maintain his slaves at a certain minimum standard of living. Excessive cruelty to slaves is also illegal, but the slaves themselves -cannot- argue it before a magistrate. Someone else has to do it for them. If the owner is then found guilty, the slave is freed and given a stipend out of the former owners assets.
Rounding up just random people and pressing them into service is also illegal, but some people just don't ask where their slaves come from as long as they can work. If a slave owner doesn't have paperwork for a slave and it's brought before the authorities, the slave is freed and a small monetary fine is assessed against the former owner.
So not -all- slavery is evil... just some aspects of it.