Not surprised that Apple didn't respond
At the present time, they'd be foolish to do so.
They have a deal with a democratically elected Government. The US has a beef with said Gov over that deal. Until the Gov and the EU come to some agreement, then it is (And so it should be) not a matter for Apple or Microsoft or Adobe or Starbucks etc etc etc.
Apple might renegociate the deal with the Eire Gov or it might move to another EU Country (queue for EU cycle to start all over again)
But this is really a matter between the EU and the Soverign (or not if you are a UKIP fan) Goverment.
Personally, I'd like the EU to win here. Then Google might not get away with their slight of hand in the UK over where their orders are placed despite their salesteams being in the UK, paid in GBP and signing the contracts in the UK etc etc etc.
A more level playing field (are you listeneing Luxembourg....) would help all round.
The companies are just trying to get the best deals for their Shareholders. Nowt wrong with that.