The answer lies in one of the quirks of the tax regime. As is shown here:
According to analyst estimates, Apple has $145 billion of cash - but only $45 billion on hand in the US, and thus not enough to fully fund the share buy-back program.
Under the US corporate tax code the profits of a company are of course taxed. That's the corporate income tax. However, profits made outside the US are, if the company declares that they're going to stay outside the US, then free from that corporate income tax. Of Apple's $145 billion cash pile some $100 billion of it has been earned outside the US. And it's being held outside the US and thus doesn't, currently, have to pay the corporate income tax.
One of the things you cannot do with money offshore in this manner is either buy back the company's stock or use it to pay a dividend. To do either of those you have to bring the money back onshore and by doing so you'll have to pay the tax. That tax is, the headline rate, 35% currently. And Apple's offshore profits have paid very little tax so far. So they would have to pay a substantial percentage of that tax if they were used to pay the increased dividend or to finance the announced share buy backs.
Thus Apple is issuing bonds which it can use to pay the dividend and so on. That offshore cash pile acts as a kind of backstop, investors aren't worried that Apple won't pay them back, but it cannot be used directly.
We've also got the announcement of the bond issues themselves:
The company intends to issue debt that includes floating- rate notes maturing in 2016 and 2018 and fixed-rate securities due in 2016, 2018, 2023 and 2043, Apple said today in a regulatory filing. Proceeds may help Cupertino, California-based Apple avoid so-called repatriation taxes on its $102.3 billion of funds held overseas as it returns an additional $55 billion to shareholders through 2015 to compensate for a stock that’s been hammered by signs of slowing growth.
The 2043 issue doesn't quite fit my narrative here but the others do. For Apple's US business alone is sufficiently cash generative that they'll be able to retire these bonds as and when they become due.
