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You have a list of items of gross income, which includes compensation for labor or personal services as an item of gross income. That is an item of income, not a source of income. If the labor was performed within the United States, then it is an item of gross income for which the source, whatever that may be, must be treated as though it is "within" the US. This says nothing about what the source is.

You missed the specific in your analysis:

Sec. 861. Income from sources within the United States
(a) Gross income from sources(activities) within United States
The following items of gross income shall be treated as
income from sources(activities) within the United States:

(3) Personal services
Compensation for labor or personal services performed in the United States;

compensation = item of income
in exchange for = source
service = object of commerce.

A tax levied as an excise or duty on an activity of commerce.

KNOWLTON v. MOORE, 178 U.S. 41 (1900)

'indirect taxes are levied upon the happening of an event or an exchange.'

Tyler v. U.S. 281 U.S. 497, 502 (1930)

An indirect tax is a tax laid upon the happening of an event,as distinguished from its tangible fruits.

PACIFIC INS. CO. v. SOULE, 74 U.S. 433 (1868),7 Wall. 433

"Congress may prescribe the basis, fix the rates, and require payment as it may deem proper. Within the limits of the Constitution it is supreme in its action. No power of supervision or control is lodged in either of the other departments of the government."

Charles C. Stewart Machine Co. v. Davis (1937), 301 U.S. 548:

The tax, which is described in the statute as an excise, is laid with uniformity throughout the United States as a duty, an impost, or an excise upon the relation of employment.

"But natural rights, so called, are as much subject to taxation as rights of lesser importance. An excise is not limited to vocations or activities that may be prohibited altogether. It is not limited to those that are the outcome of a franchise. It extends to vocations or activities pursued as of common right."

Employment is a business relation, if not itself a business. It is a relation without which business could seldom be carried on effectively. The power to tax the activities and relations that constitute a calling considered as a unit is the power to tax any of them. The whole includes the parts. Nashville, C. & St. L. Ry. Co. v. Wallace, 288 U.S. 249, 267 , 268 S., 53 S.Ct. 345, 349, 350, 87 A.L.R. 1191

Looks more than sufficiently specific in regards to the intent of Congress under authority granted by the Constitution which is the rule.

Constitution for the United States of America:

Article VI: "This Constitution, and the laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the Constitution or laws of any State to the contrary notwithstanding."

Article I Section 8: "The Congress shall have power to lay and collect taxes, duties, imposts and excises,
to pay the debts and provide for the common defense and general welfare of the United States;
but all duties, imposts and excises shall be uniform throughout the United States; "

Article I Section 8: "To make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof."

House Congressional Record, March 27, 1943, pg. 2580:

"The income tax is, therefore, not a tax on income as such. It is an excise tax with respect to certain activities and privileges (the type 3 and 4 taxes) which is measured by reference to the income which they produce. The income is not the subject of the tax; it is the basis for determining the amount of tax."

FindLaw: RODGERS v. U S, 185 U.S. 83 (1902)
"The primary rule of statutory construction is, of course, to give effect to the intention of the legislature."

FindLaw: S.E.C v. C. M. JOINER LEASING CORP., 320 U.S. 344,351 (1943)

"... courts will construe the details of an act in conformity with its dominating general purpose, will read text in the light of context and will interpret the text so far as the meaning of the words fairly permits so as to carry out in particular cases the generally expressed legislative policy.

So your problem with the language is what? Seems totally clear to me.