Here’s more from David Schmidtz’s Elements of Justice, in which he is engaging Rawls’ thought experiment on original position that presumes a closed society as the basis for his social thought. In a closed society we only enter by birth and leave by dying. Schmidtz observes that

as a matter of historical record the least advantaged have always been better off in open societies, societies where people are free to move in search of better opportunities. if we are theorizing about what kind of society is best for the least advantaged – if that is the desired conclusion – then is anything more fundamental than the freedom of movement? Indeed, why not deem freedom of movement the core of the first principle: Everyone has a right to live in a maximally open society, a society where they have no obligation to stay if they would rather be elsewhere? (222)

My guess is that Rawls is concerned with describing a grand (perhaps utopian) global vision for human society, which ultimately is closed and in which migration wouldn’t be of consequence. But Schmidtz is right to point out that practically that vision is not within our grasp, and is of little use paring the various actual different human societies.