(Note: I am neither of the AC ancestors, but I'm pretty sure I understand their position, so I'll try to explain it regardless)
The critical difference here is that those Chinese workers are /not/ slaves. They are not forced into taking jobs at foxconn; they take these positions voluntarily, just like people in western countries do, because they think it's a favorable trade for them.
Why do they do this? Because as bad as the working conditions and pay at companies like Foxconn are by western standards, they are very competitive compared to the local alternatives. This point is crucial: Foxconn are not exploiting people in the sense that all else being equal, the people who work for them would be better off just not doing so.
You can make an argument that people living in sufficient poverty to make such a deal favorable is a terrible thing, and I'd agree with that. However, destroying Foxconn's business model by preventing them from selling to western countries does nothing directly to fix these people's poverty; in fact it makes it worse, by reducing the pool of jobs available to them (and not just randomly reducing it; you're taking away some of the best jobs in the pool!).
As an analogy, think of how you'd react if people in a hypothetical country that's even more wealthy than your's decided that your working conditions are far too horrible for your pay, and somehow stopped jobs like the one you have right from being offered anymore, resulting in you having to choose a worse job instead. Would that make your life better? Would you be happy about it? It's the same thing here.
The above is how the simple economic argument goes. Real economies and societies are complicated, of course, and there's several vectors by which driving Foxconn out of business oculd potentially improve the situation for common workers in China. But those aren't clear to me (and aren't clear to various other people who've looked at the issue) - the direct, obvious and robust effect is strongly negative. If you're going to argue that there are other effects compensating for it, it would be good to present your reasoning or link to other people arguing for the above reasoning being incorrect.