It could crush the economies of poor nations by making their exports unaffordable in wealthy nations (and the rebates funneled through NGOs wouldn’t compensate for that harm).

This is a circular objection, but nevertheless valid. These countries are poor because their labor isn’t valued. Trade is about comparative advantage. Poor countries that trade have a comparative advantage in one thing: how crappy a life their people can be made to live without rebelling, leaving, or killing themselves.

Poor countries are already “crushed,” by poverty. Those countries have rich people whose wealth arises from the oppression of everyone else there. Their populations consist of the thugs who run the place and those who lack the wherewithal to leave. Recall that powerful Africans were complicit in the slave trade. The bosses there literally sold the suffering of their people. Today, the physical transport is unnecessary, and the slavery is de facto rather than de jure. But the economics — and the morality — are the same.

The founders of the US understood the importance of coordinating interstate commerce. We have some labor laws (not enough, but some) that preclude states from competing on the basis of their own poverty. From abolition until recently, the technology of transportation and communication made trans-oceanic trade in misery difficult. But things change. Globalization demands that the entire world operate as a common market with respect to the exploitation of labor.

Politics is just economics continued by other means. The US has one national government; the world most assuredly is not ready for that.The economic simplicity of the EEC inevitably morphed into the political mess that is the EU. (Students of American history look at the EU and think “Articles of Confederation,” a half measure that could not solve the binary problem of cohesion.)

Just as a US citizen cannot sell his labor for $1 per day, countries must not be allowed to trade on their people’s misery. If the markets of the world unite to eschew the products of misery, maybe the purveyors of misery will be forced to invest in something more sustainable. But it just may be that some places are uninhabitable unless the people there sell their misery for subsistence. Those place need to be migrated from, or, to prevent having to receive their masses, the industrialized world must impose prosperity (education, rule of law, good government) upon them, hardly an easy task. No law of nature requires that this end well, and history suggests that it probably won’t. But M. Payre has the right idea.