The point is that Apple is not just operating in the USA; it is operating around the world and makes profits around the world. No one disputes that Apple pays U.S. taxes on their U.S. operations, and no one has accused them of moving U.S. profits overseas to avoid taxes. The big to-do is entirely about their earnings overseas.

The question of the ethics of international taxation is complicated by the fact that most countries in the world do not expect their companies to pay both local taxes and domestic taxes; the U.S. does. So it is not like Apple is violating some broad standard of behavior. If anything the U.S. tax system is the aberration.