So if the EU starts getting directly involved in tax for those companies in Ireland it directly impacts the tax revenues in the US.
The USA has low personal taxation but high corporate taxation. Most other countries, including virtually all EU countries, do the opposite.
Furthermore, unlike most other countries, the USA taxes dividends received as ordinary revenue at high tax rates, even though they have already been subject to corporation taxes. The taxes in Ireland are not levied on the US part of Apple. They are levied on Apple Europe. If Apple Europe were to pay those taxed (however low) profits to the USA, they would be subject to punitive US corporation taxes on Apple USA. No wonder Apple prefer to not pay dividends to its US parent, but luckily companies can’t be forced to pay dividends. However the USA can always make its tax code even more punitive by begin levying taxes on unpaid dividends.