I recall watching a documentary about the way coffee is purchased and sold. If I remember right all of the coffee was purchased by the large companies like Nestle' from a central point where all of the farmers bring the beans to. This would mean that no one really knows what plantation the beans came from by the time they get purchased. The slavery is terrible but I don't think that Nestle' or any of these other large coffee companies are responsible for it so I don't think finger pointing will solve anything. This is an issue that needs to be taken care of on the ground where it's happening.
That doesn't seem surprising at all. Agricultural stuff, especially when it comes from poorer countries, can be really hard to figure out. Even something like fair trade can actually be quite controversial. Fortunately, it seems at least when it comes to slavery, there are human rights organizations willing to tackle the issue.