The US has Constitutional jurisdiction over "income" earned anywhere in the world by anyone.
Technically, Congress has not imposed a tax on "income."
Rather, it has imposed a tax on "taxable income," which is a term of art that Congress created and defined.
See: 26 USC 1.
When you research that, you get to the definition of "gross income," and when you research the regulations, including the past history of the regulations regarding "gross income," you find that the regulations say that gross income means ... blah blah blah ... UNLESS EXCLUDED BY LAW.
This does not appear in the statute, but only in the regulations.
The past regulations said, "... unless excluded by fundamental law, or otherwise not taxed under the Constitution." This language has been changed over the years to hide the true meaning, but the meaning still exists because it MUST exist in order to stay constitutional (and to be in line with the SCOTUS decisions about what "income" means).
There are also some areas of the tax code that give a different definition of "income," which are very interesting, as well.
Technically, Congress has not imposed a tax on "income."
Rather, it has imposed a tax on "taxable income," which is a term of art that Congress created and defined.
See: 26 USC 1.
When you research that, you get to the definition of "gross income," and when you research the regulations, including the past history of the regulations regarding "gross income," you find that the regulations say that gross income means ... blah blah blah ... UNLESS EXCLUDED BY LAW.
This does not appear in the statute, but only in the regulations.
The past regulations said, "... unless excluded by fundamental law, or otherwise not taxed under the Constitution." This language has been changed over the years to hide the true meaning, but the meaning still exists because it MUST exist in order to stay constitutional (and to be in line with the SCOTUS decisions about what "income" means).
There are also some areas of the tax code that give a different definition of "income," which are very interesting, as well.