They never pay the slaves enough so they can get free, just enough so they can stay alive and come back to work. I could see all this. Why couldn't they?
It's an interesting aspect of Capitalism: Not only does competition keep prices for products down, but on the other end, competition for jobs keeps the wages down as well. Overall wealth usually grows, but free time does not. By all accounts, we have mechanized enough that we should be able to produce far more for each if we were able to organize in a more-efficient fashion, but currently, we cannot.
I don't buy much into the notion of one class oppressing another, so much as there are people playing roles in a sustainable system. We should always search for better sustainable systems, but in a thoughtful, not wishful way.