So they create a race of beings to do all the work for them. The more the merrier. We not only do the mining, we do the refinement. Introduce capitalism and we have a boom. More people means more resources, but the original miners don't care how they get their product. They could gather up every man-made item on this earth, like a huge vacuum, and all they have to do is throw it in a giant recycling plant and they cart it off. We did all the work and they threw us a few scraps to keep working for them. We are like worms, using every resource on this planet for our masters. That is, if there really are aliens mining Earth.