McCaskill's question is great.
So if Apple is really just about minimizing tax burdens as a matter of doing what you can within the law to maximize profit for your shareholders -- as they say, and as their defenders here say -- why not shift the company entirely off-shore?
Cook had two options: admit he was screwing his shareholders by not doing that, or admit that there's more to it than just maximizing profits. He choose (b) -- "we're an American company" -- but couldn't explain why it wasn't unAmerican to hide 70% of your profits from America..
I'll explain it on his behalf. Profits that are not earned in America aren't subject to taxation by America. Profits are subject to taxation in the country where they are earned, so even if Apple relocated overseas it would still be subject to taxes owed on account of their U.S. sales, just as Samsung is today. And, as you may know, most of Apple's revenue is not generated in the U.S.
Unfortunately, it takes a good bit of study and experience to fully understand the complexities of taxation of global enterprises, both from the perspective of the world's taxing authorities, and from the point of view of the muli-national corporations. There is nothing simplistic or intuitive about any of it, and only the most naive would attempt to address the issues from some simple chauvinistic or moralistic starting point.
For example, many countries are far more interested in generating economic activity to provide jobs (which in turn provide a tax base with a multiplier effect, as well as to support social stability) then they are in collecting corporate taxes. . Such a government might well forego taxing a company that wishes to build factories and stores and to offer construction and retail jobs to its citizens. In the end, such countries believe, they will garner more overall tax revenue while creating a healthier and more productive society.
That a company like Apple pays no taxes to such a country creates no obligation--legal or otherwise--to pay any more tax to the country where it happens to have its headquarters.
Were the U.S. to require taxes to be paid on non-repatriated foreign income earned by companies headquartered in the U.S., there would be far fewer companies with overseas operations headquartered here, and far less economic activity and jobs in the U.S. The world has become much more competitive, and America is no longer such an advantageous place to be that it's worth substantially diluting global shareholder value for the privilege of having a +1 in front of your main office phone number. Congress knows this very well, and while some may grandstand for the cameras, their staffs will point out how self-defeating short-sighted corporate tax policy could be.
Campaign contributions, bribes, or currying favor with the rich, are silly explanations for our tax laws. They are written as they are because it is in the best interests of America to balance the need to maximize overall tax revenue against encouraging the maximum amount of domestic economic activity.
Apple certainly doesn't need to apologize for obeying the tax laws of every one of the countries it sells or operates in, and Congress doesn't need to fix corporate tax law to address any glaring injustice.
Whether some politicians need to apologize for stirring up a largely uninformed public at the expense of a law-abiding corporation that has contributed six billion dollars to the Treasury while providing directly and indirectly jobs for hundreds of thousands of Americans who also pay taxes is quite another question.
