Many of the outer planets, most notably Persephone, actually have a highly slave-based economy. Much as Independent propaganda likes to diminish this, they tend to abduct a huge amount of people from less developed planets and moons and put them to work in farming and textiles. There are also some unconfirmed rumours that, shocking as it might sound, they actually present scores of them as Reaver tribute, but as regards everything Reaver-related, too much of it is dependent on hearsay, and we're not even sure if these supposed humanoid beings are even real, and it'd be wise of you not to pry either.