As usual, Alex Tabarrok at Marginal Revolution illustrates Across Difficult Country's aphorism. Alex says:
I understand individual rights and I understand counting everyone equally but I see less value in counting some in and some out based on arbitrary characteristics like which side of the border the actors fall on.
The difference is quite obvious if you remove the libertarian economists' assume-we-have-a-can-opener blinders. We live in a world where violence -- perpetrating it and preventing it -- is the fundamental fact that social and political organization must deal with.
Thus, all property rights come out of the barrel of a gun.
Once you realize that, the reason why we prefer the welfare of our fellow citizens to that of non-citizens is (to get all reductionist):
They are the ones who would fight on your side.