The US operates under world-wide income
http://www.irs.gov/Individuals/International-Taxpayers/U.S.-Citizens-and-Resident-Aliens-Abroad but under US law it doesn't matter where you live.
For citizens of Germany it's world-wide income as well...
As I pointed out earlier, the double tax treaty is to facilitate payment of bills, not determine bills. As an example my tax is accessed in both the UK and Germany under the law of both countries. The treaty ensures I do not pay the same bill twice. How the bills are arrived at is independent of...
As I understand it, this is to facilitate payment of tax bills, not determine tax bills. The idea being you do not pay the same bill twice. Boris will have two separate bills.
There is no 'fault.' It's just a different way of doing things. If he does not like it then he could always relinquish his passport. He has dual-nationality so no one is forcing him to keep it.
The rules for US passport holders are different. As I understand it US passport holders have to pay tax above a threshold even if they are not resident in the US. So they are in effect taxed twice. The UK does not do this. But, as it happens, I do pay a small amount of UK tax on some UK earnings...
http://gu.com/p/43f85
This could get very interesting. If he hasn't been paying any US tax, which is what is being suggested, his back-dated bill will be huge.