Interesting, so you are equating the actions of an individual here with a corporation. At least you are playing fair in that respect.
Slave labor, to me, is half-freedom (you can buy from anywhere, but the suppliers coerce work out of their workers). If you are talking about very low pay, but the workers are free to leave, then they are choosing it over the alternative (which they are determining for themselves to be actually worse).
When the government of Fiji comes in and says the fishing companies have to pay for free health care and the companies relocate and everyone loses their jobs (true story) the workers are the ones who lose.