Starbucks will not have wanted to be using slaves, they only pay the workers 2¢ on the cup anyway. It is a fairly minor expense in the greater scheme of things.
What it is, is an emergent risk in extended labour chains (or "cascades"). Particularly prevalent in harvest work. At some point your supply chain transparency breaks down, how ever many steps of outsourcing deep that might be.
Unsurprisingly the gangmaster not actually paying his workers is likely the lowest bidder, so in the cruellest sense of the "free" market, every company wants to use slave labour. But to a global business with at least some accountability, this is a massive fuck up in oversight.
You could think of it as stochastic slave trading, if you wanted to over-intellectualise it. Certainly oligopsonies generate market pressures that strongly incentivise the emergence of modern slavery and labour abuse in the supply chain.