I will note though that New Zealand is part of the tax planning strategy that Google uses to save. They've recently been destroyed by the PR machine for doing this, but its honestly pretty common. I'm constantly hearing and learning about new ways to leverage laws between federal, state and international tax rules to do the same.
I think a dual tax agreement is a bit different than a low-tax IBC generally is a situation where you dont have local taxes and you pay small fees for doing business (franchise tax). The tax havens in the bermudas and carribeans do this a lot.
A dual tax agreement is usually an agreement between the two governments saying that, they will give a tax credit for taxes paid to the other guy so that the same amount of income isn't taxed twice.
The necessary piece of information here that most people don't make explicit is that US persons (natural or companies) are liable for all income worldwide (incl. from foreign subsidiaries when they bring that cash back to the US); the thing Google is doing just deferring that day by holding the money offshore. In practice they can defer indefinitely, and use the overseas cash to finance overseas operations, make overseas investments, etc.
There are cases (Barbados!) where due to awesome double tax treaties, you can actually end up paying less US tax under a double tax treaty than in a zero tax jurisdiction subsidiary -- mainly by using the foreign entity in the double tax jurisdiction to recharacterize earnings as capital gains or dividends. There was a period where dividends were not taxed much. Even when magic loopholes like that only exist for one year, it becomes a big deal because these pools of offshore deferred earnings can all stream through during that window.
