It has no such moral claim - but it has a claim nevertheless. Likewise with monies earned overseas; the US and Eritrea are the only two nations that tax based upon citizenship, not locale. The US makes a claim to taxes on any money you earn anywhere in the world, if you're a citizen or permanent resident. Highly immoral - but a claim nevertheless.
As far as income taxes go, I'm not sure they are inherently immoral - but I do consider non-flat income tax rates as immoral. If we're going to tax income, then it should be a single flat rate that all pay, regardless of amount of income.
RE: the US and Eritrea are the only two nations that tax based upon citizenship, not locale
Interesting. Can you share with us your sources for this information?