Of course it is valid. The principal on which much of taxation is based is that it is a tax on the benefit that the income provides. And that benefit accrues in the US.
The only way it can be claimed to be invalid is by conflating legal with moral.
You're suggesting that their EU sales shouldn't pay EU income at all, rather US income taxes instead? Usually the criticism of this is the opposite direction, that sales in Germany should pay taxes in Germany.
That used to be true, but is no longer. Currently Apple et al pay 12.5% corporation tax in the Republic of Ireland, unlike the past where this rate existed but was notional as most of the revenue was ultimately booked to a real tax haven like Bermuda et al.
