Let's quote a paragraph here:
For purposes of subparagraph (A), there may be excluded from consideration -
(iv) employees who are nonresident aliens and who receive no earned income (within the meaning of section 911(d)(2)) from the employer which constitutes income from sources within the United States (within the meaning of section 861(a)(3)).
Congratulations, you just proved my point right there.
You still haven't indicated where in the statutes or regulations it says that US citizens should ignore Section 861 for determining gross income and taxable income from sources within the United States, or where that section is only applicable to aliens, and I've posted a number of indications that point every "taxpayer" to that section. Unless you're suggesting that the term "taxpayer" as it is used in 861 and its regulations only refers to aliens...