The basis upon the belief that there is NO such thing as an "income" tax are several sources.
Another problem, is that in Section 61(a)(1) it states "Compensation for services, including fees, commissions, fringe benefits, and similar items;" the most notable abscences are salaries or wages. In section 871(a)(1)(A), it specifically states "interest (other than original issue discount as defined in section 1273), dividends, rents, salaries, wages, premiums ...". Why in Section 871(a)(1)(A) does it specifically state salaries and wages, but in Section 61(a)(1) it does not?
The US Constitution, article 1 section 9 states "No Capitation, or other direct, Tax shall be laid, unless in Proportion to the Census or Enumeration herein before directed to be taken." Since income tax would fall within "other direct Tax", it has to be apportioned. Since apportionment is EXTREMELY difficult (however not impossible) for the US Government, the Government had to come up with a way to confuse the citizens into believing they have to pay an income tax. Also, since the word income is used in the 16th Amendment Congress cannot define "income" since that would alter the US Constitution by legislation alone, and they cannot do that.
Section 871 applies to US Citizens working abroad, not connected to a US business. Section 61 applies to US Citizens working within the US for a US business. Since the US Constitution does not apply outside the United States, the 16th Amendment also does not apply. Since the US Constitution does not apply, the Government can collect taxes ON salaries and wages of persons working abroad, without the burden of the US Constitution's apportionment considerations.