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.
Of course, in practice there's a lot of leeway in deciding how much attention one should pay to customers; loyal customers are key to a company's success.
Whether or not to pay extra taxes, however, is cut and dry. There's virtually no benefit to Apple in voluntarily paying a higher tax, so it doesn't do so. Any obligation it has to the state must be explicitly stated in law. There's no concept of civic irresponsibility for companies; that's exactly what laws are for. there's only legal and illegal.