Many corporations have turned to offshore tax shelters to skirt their tax obligations in the United States. This can be done by a number of methods, the most popular of which is incorporating in other country. 60 Minutes did a story in which they attempted to visit the "world headquarters" of some multi-national corporations, only to find there were no executives there at all.
Offshore tax sheltering causes the US government to lose revenue, but it is much harder to pin corporations on this because they often don't break any laws. Personal tax shelters are undoubtedly illegal, but a multi-national corporation can technically choose their "residency." Corporations will choose countries that have little to no corporate tax, and end up saving would-be taxes into the billions of dollars.