Ethical, to have a meaning in the common sense, has to refer to "the society as a whole" -- not to whom you have a "responsibility to".
A gang member also has a responsibility to his gang members (e.g to maximize profit and get rid of people causing trouble there too). That doesn't make his crimes "ethical".
So, no, it can be legal and not ethical. Finding corner cases to side-step the spirit of the law is not regarded "ethical" in any place I know of.
A better metaphor would be a U.S. citizen who takes the IRS standard deduction even though they know that their itemized deductions would be less. Is that unethical?
What about people who use TurboTax software, which helps them find deductions that they would not have otherwise known about? Is that unethical?
In addition, consider the society of nations as a whole. The U.S. alone among developed nations expects its companies to pay U.S. taxes on profits earned globally. If Apple were a French, Dutch, English, German, etc. company then this conversation would not even be happening. They would just pay the local tax rate and then repatriate the remaining earnings.
The problem is that it is not always clear cut what is legal and what is illegal when it comes to taxation on an international scale.
Tax laws are often written in language that lays out principles - such as profit should be booked in the country that the sale takes place in - but it is very easy to manipulate "profits" to make them appear in other countries. If a company goes too far to manipulate profits out of a country with relatively high taxes it can become illegal - but it would take a full investigation to determine this.
More certainly can and should be done to close loopholes and improve coordination between countries, but it is not a simple matter of a country writing "a new law" to fix the problem.
On topic, it's funny that the Congress seems so eager to treat Apple as a one of a kind bad boy whereas most of this industry is doing the same, and would be stupid not to. They bring more to their country than if they were blindingly paying the taxes the Congress think they should pay: more money means more products and more jobs. Simplistic view of the whole thing, I admit, but if France has shown one thing, is that we don't gain competitivity by suffocating companies with taxes.
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.
Please stop saying that. It's urban legend repeated by those that wish to excuse any corporate behavior.
(I'm more than willing to change my stance as soon as someone provides a link backing up the "legal obligation" argument.)
Apple's responsibility is to its shareholders, and that responsibity is to maximize profits. Because it stands to lose billions of dollars in taxes if its income is reported naievely, it makes perfect sense for Apple to spend millions, tens of millions, or even hundreds of millions to optimize its tax flow.
As long as nothing illegal is happening, this is just an exams of a company fulfilling its fiduciary duty to its owners.
If people think this behavior is egregious and needs to change, the solution is to pass new laws to that extent. No one is going to voluntarily overpay on their taxes.