First, back up to the question of the justification of a system of private property. The division of the commons into parcels, and the use of government coercion to enforce private claims over these parcels — which include the right to exclude — requires a justification. Dave Schmidtz provides that justification here [doc]. In short, dividing the commons leaves each with more than had it remained open. The right to exclude enables general prosperity.
So, think of the Earth as a big commons, and imagine borders as fences. Can we justify the system of nation-states and its migration controls in the same way? Evidently not. The welfare gains that would come from even a mild decrease in coercive limits on travel and free association are awesomely huge, which of course implies that the status quo system of limits does not leave most people better off than they would be in a feasible alternative system. And this suggests that the global-level system of division and exclusion lacks moral justification.