RE: the US and Eritrea are the only two nations that tax based upon citizenship, not locale
Interesting. Can you share with us your sources for this information?
Also, as a guy who's worked internationally a few times - and has several acquaintances and friends here in the US who are from foreign nations - I've confirmed it first-hand. My friend from Germany (and still a German citizen) who works at Microsoft doesn't pay German income taxes; only US taxes. But when I worked in Germany for 9 months I paid both US and German taxes.
We're pretty unique in that regard - you earn a dollar anywhere in the world, and even if you don't step foot in the US for the entire year, the US still wants a cut of your income - or proof that you gave taxes (at least equivalent to what you'd pay in the US) to someone else, on your first $107,000. Above that point - you get to pay foreign and US income taxes guaranteed.
Oh, and if you spend more than 35 days a year in the US, you don't even get to deduct those foreign income taxes you paid. Full double-taxation - your foreign domicile and US. Better plan your business trips and vacations back to the US with care, because staying here 1 day too long could cost you $30,000 in taxes.