Well, underpaid by what metric? If each Chinese laborer produces $100/day in profit but only gets paid $10/day, morally they're being underpaid. And the fact that so many companies are still moving operations to China is itself evidence that on average more profit is allocated to capital holders when they use Chinese labor over, say, Californian.
But even if you take liberal/market ideology at face value, they're still being underpaid. To have a free market you've got to have free people. If you agitate for more workplace protections or (god forbid) try to start an independent labor union, getting shot or sent to a prison camp in the far western provinces of China isn't out of the question. This decreases labor's bargaining power relative to what it would be in a free market, which decreases wages.
Ok, I accept your second point. Regarding the first... If there was an unskilled labour that, by itself could produce $100/day profit for a majority of people, then you wouldn't have any $10/day labourers. This $100/day profit is only possible with the value added by the skilled labour. It is like this everywhere. For example, the artist who made the facebook logo only got paid in the hundreds of dollars yet the facebook logo produces much more than 10x that in profit. Yet, without the value that the facebook product adds to the logo, it would have been worthless. You wouldn't argue that the artist is underpaid.
They don't produce that much profit in China. Companies compete all the profits away, and the big profits in many profits are overseas. Which is not to say they are not necessarily underpaid but it is not a simple calculation.
But even if you take liberal/market ideology at face value, they're still being underpaid. To have a free market you've got to have free people. If you agitate for more workplace protections or (god forbid) try to start an independent labor union, getting shot or sent to a prison camp in the far western provinces of China isn't out of the question. This decreases labor's bargaining power relative to what it would be in a free market, which decreases wages.