Re: American citizens abroad
If you're a U.S. citizen it doesn't matter if you earned your income on Mars. You pay U.S. taxes on it. There may be some deals where if you pay foreign taxes you get some kind of break, but I've never dealt with that part of the tax code.
Anybody eligible to pay income tax will be paying payroll taxes in some form. I'm self-employed, and pay not only my half, but the half usually "invisibly" paid by the employer.