I may be wrong but my understanding is US taxes the difference between foreign and domestic tax rates.
Re:Apple's cash hoard swells to $246B
by DanielBMarkham on Sunday April 16, 2017 @08:30AM
(#14125546)
Also once they pay tax (again) on it to get it into the U.S., it gets taxed a third time if they distribute it to the shareholders.
To clarify: they haven't paid US taxes on it. They've paid whatever taxes are levied by the foreign countries they're operating in, but the US also double-taxes that same money if it's brought back into the US.