US "favorable corporate tax structure"
Apple pays about a 30% rate on the money they make in the US. If they brought the overseas cash in, they'd pay 35% the amount, less whatever they've already paid on it overseas. They are and for the last several years have been the single largest taxpayer in the United States. Larger than any bank, oil company, or billionaire investor.
A company like Apple doesn't have much room for deductions, credits and loopholes. Look at the effective tax rates for companies like Walmart, that pay pretty much the full whack, versus banks, which pay very low rates and sometimes get multi billion dollar refunds. GE is the master of working the US tax system, they make billions every year but pay almost nothing in taxes. Why? Because they've been around forever, lobbying congress forever, and got favorable loopholes inserted for them. Apple hasn't been lobbying congress for loopholes in the US code, so they don't have any aside from general stuff like the R&D tax credit.
The reason for this whole mess is because the US taxes money made overseas, and hardly any other countries do the same. But because they don't make the tax due until you bring it home, it provides a perverse incentive for companies to leave the money overseas forever - hoping for another tax holiday like the one in 2004 where the money could be brought home at 5.5% or something like that.
If taxes were due on money made overseas in the year it was earned, like it is for US income, companies wouldn't go to such great lengths to avoid taxes overseas, so long as the US tax rate was higher. What would be the point of complex structures to lower your overseas tax liability when you'd pay your savings to the IRS, dollar for dollar? That's what the other countries should lobby the US to change.