If the person voluntarily agreed to be bound by X terms, and X terms are followed, then there is no moral problem.
Except that this only happens in situations where prevalent working conditions in a society are so bad that they are effectively forced into slavery. Either that or starve to death. Well, it's technically a choice, right?
Libertarian ideas of "freedom" are scary as fuck.
And what is your solution? To not allow desperate people to make the choice in the first place?
What exactly is the alternative you are advocating? When a sweat shop is closed... who is helped?