It is efficiency, really, because it's making better use of an underutilized resource. Over time, of course, the price of labor evens out because the resource is no longer underutilized.
> more a question of fairness
If it's fairness you want, telling companies to hire locally isn't going to cut it. That's the opposite of fairness, really—you would be subsidizing local workers whose wages are already higher at the expense of foreign labor that has more need for the income.
> Another potential advantage could be, factories are closer to natural resources, but it seems to me that that's not as much a question of efficiency, as it is of lax (enforcement of) environmental standards.
Locating factories near their required materials is a net gain even when ideal environmental standards are maintained. It's almost always cheaper (economically and environmentally) to ship end products where they need to go than it would be to ship raw materials around to distributed factories. Concentrating manufacturing in one area also makes it easier to confine the environmental impact.