I might be wrong here but aren't most US companies flush with cash these days? I would assume that they use their cash reserves for the buybacks and not debt. Are there any stats on what is used to finance the buybacks?
I'm pretty sure they don't do this as a loan from the untaxed or under-taxed overseas entity back to the taxed parent would create a tax event.
AAPL has more cash than it knows what to do with, but it's a huge borrower in the corporate market so that it can fund its dividend payments.