Over the years, Apple Inc. has become the poster child for U.S. multinationals accused of sheltering overseas profits to avoid the IRS. What's gone largely unnoticed is that it's been paid more than half a billion dollars by the U.S. government to do just that.

Taking advantage of an exemption tucked into America's Byzantine tax code, Apple stashed much of its foreign earnings—tax-free—right here in the U.S., in part by purchasing government bonds, according to people with direct knowledge of the matter. In return, the Treasury Department paid Apple at least $600 million and possibly much more over the past five years in the form of interest, a Bloomberg review of its regulatory filings shows.

The untold story of Apple and its taxes wends its way from Cork, Ireland, to New York and then Reno, Nev. But according to tax experts interviewed by Bloomberg News, the maker of iPhones is hardly unique. Many of the biggest U.S. multinationals have seized on the same exemption, which lets them avoid or delay repatriation taxes by buying Treasuries with their overseas cash. (The top 10 alone hold over $100 billion of the bonds.) That, in effect, enables the companies to turn billions of dollars in potential tax liabilities into millions of dollars in taxpayer subsidies—all while they publicly bemoan the sky-high taxes that make it impossible for them to bring the money home.