It seems that UBI arguments are all about "fairness". So it naturally should extend to other countries it seems. Otherwise you are just creating another greedy / protected group.
Of course people usually try to draw the UBI Venn diagram such that they are a net receiver of funds.
What would be absurd about a global UBI? It's amazing how fast people jump off the high horse of equality when you point out that on a global scale they are incredibly rich and privileged.
Equality to them means them getting more material goods, not them giving up more material goods.
It's about impracticality, not morality. It doesn't make feasible sense to fix the whole world's economy in one go. And we shouldn't let imperfection get in the way of progress.
It is hard to say "I want UBI because inequality" and then fail to recognize this.
What they are really saying is "I don't want anyone to be richer than I am but fine with people being poorer". So the default human position on things.
