It's voluntary because they can chose the second best option: Farming, fishing... And the second best option would have nothing to do with F21 or any other company that uses sweatshops. If you look at it from that vantage point, F21 is giving them a new option that they didn't have before. However, that's not the point I'm trying to make, since I believe that sweatshops have negative externatilities that I haven't accounted for.
Choosing anything other than the best option is irrational. Why would I choose second best over best? It's not really a choice, just a word game to rationalize disturbing facts.
I don't understand your point. You are arguing that working for 21F is the best option, yet you oppose companies opening up sweatshops in developing countries?
How is it voluntary (note, even you had to use quotes around this word) if it's the best option they have and/or they have no other options?
Just because some humans in some part of the world at some time did something, does not mean that many other humans also have to do it.