Upon further research, this whole issue is about repatriation of income. The U.S. tax system works like this:
1. Non-U.S. companies are taxed (up to 35%) based on earnings in the U.S. They do not pay taxes on foreign income that is brought to the U.S.
2. U.S. companies are taxed (up to 35%) based on earnings anywhere in the world. However, they can delay the tax on foreign earnings if they choose not to bring the money back to the U.S. They are also given a tax credit based on foreign tax paid.
Let's apply the above to hypothetical, assuming Walgreen's operates stores in Switzerland and Switzerland has a 20% corporate tax rate.
If Walgreens is a U.S. company, it pays 35% on U.S. earnings and 20% on Swiss earnings. Then, when it transfers the Swiss earnings to the U.S., it pays the U.S. an additional 15% (35% - 20%). If Walgreens chooses, instead, to use the Swiss earnings outside the U.S., it pays 0% on their earnings.
If Walgreens is a Swiss company, it
still pays 35% on U.S. earnings, and it still pays 20% on Swiss earnings. However, if Walgreens chooses to use the money earned in Switzerland to expand its U.S. operations, it pays the U.S. 0% on those earnings, instead of 15%.
Here's an article explaining the above briefly:
http://taxfoundation.org/blog/basic-lesson-us-s-corporate-income-tax-system
And here's the organization that actually produced the report on which the story was based:
http://www.americansfortaxfairness.org/walgreens/
If you read the official report, it is relying on estimates by reputable companies of Walgreen's anticipated one-year tax savings, and then extrapolated that to $4 billion over 5 years. However, the reference articles are by subscription only and the report makes no effort to explain how the companies estimated the 1-year tax savings; probably because an explanation that Walgreen's is still paying full taxes on its revenue from U.S. stores would undermine the purpose of the report.
edit: In a way, this article is actually about the IRS policy of discriminating against U.S. citizens by charging them more in taxes than they charge non-U.S. citizens for the exact same activities. Granted, that might make sense for an individual citizen, who has additional benefits that non-citizens lack (like the right to vote), but there is no such additional value in being a U.S. corporation.