> Because they'd have to pay taxes if they use the foreign reserves?
This is the main reason.
> Because they are somehow getting a higher interest rate return on their reserves then they'd have to pay to borrow? (That would seem pretty impossible).
This is also a possibility. Apple has extraordinarily good credit. They can consequently borrow at near-zero interest rates. They can then invest that amount of money in some (perhaps only slightly) higher risk security than their own bonds and turn a profit -- and if the money was originally held by a foreign subsidiary then so is the profit from the investment.
This is the main reason.
> Because they are somehow getting a higher interest rate return on their reserves then they'd have to pay to borrow? (That would seem pretty impossible).
This is also a possibility. Apple has extraordinarily good credit. They can consequently borrow at near-zero interest rates. They can then invest that amount of money in some (perhaps only slightly) higher risk security than their own bonds and turn a profit -- and if the money was originally held by a foreign subsidiary then so is the profit from the investment.