The EU is applying the back taxes to worldwide profits, as Apple is choosing to transfer those profits to the EU
Apple doesn't transfer U.S. profits to to the EU, so how is it fair for the E.U. to tax Apple on U.S. profits again exactly?
That, kids, is why they play this game.
There are $14 billion reasons why the EU is playing this game but legality or fairness is not one of them.
Ireland does not charge taxes for profit made in other countries. Apple transferred their profits to Ireland to avoid paying taxes in other countries. If they transferred US profits to Ireland then, yes, they do have to pay tax on it. This is because of the way that Apple transfers the money - as a patent licensing fee. The subsidiary in Ireland is making a profit on all those fees they paid to Ireland worldwide.