It's not so much about altruism or the moral validity of taxation. It's about equality and fairness. If some people can take advantage from the expropriation of others, then who gets to say who can be expropriated and who can't? With taxation the justification comes more or less from the revenues and wealth differential, basically it is assumed that rich people can pay because they suffer less from the requisition of a small part of their wealth. But if the criteria is only nationality, I'm not sure they will accept it that easily.