As far as I have been able to tell from the behaviour of those who believe this kind of thing, the correct authority to consult to determine ethical tax liability is the sub-editor in charge of writing headlines for whichever newspaper one reads.

You know, there are things that are legal, but you know are unethical and you don't do them. Refusing get up to let an elderly woman sit on the bus is not illegal, but you do it because it's the right thing to do.

Apple go to great lengths to make their taxes 0, doing things that small businesses would never be able to do. If they didn't their taxes would be hundreds of millions. This—alongside depriving California and the States of money that could be used for the community—makes it unethical IMHO.