It's probably a sub section of the law that requires payment of federal income tax.
I realize this is OT, but this has driven me mad for years. The clearest way I can see the system works is this:
1) The tax code requires employers to withhold taxes:
(a) Requirement of withholding
(1) In general
Except as otherwise provided in this section, every employer making payment of wages shall deduct and withhold upon such wages a tax determined in accordance with tables or computational procedures prescribed by the Secretary.
http://www.law.cornell.edu/uscode/text/26/3402
2) Then the tax code requires the individual to rectify the accounting by requiring the filing of a 1040 return. Sorry but I'm not sure where this section is atm. ETA: this may be part of it, but this section applies to a "person" (not an employer or employee) whereas if one receives wages from an employer, one isn't then liable to pay the tax per the above language:
http://www.law.cornell.edu/uscode/text/26/7203
3) The code also defines "employee". On an initial read it sounds like it only applies to employees of governments, but upon further inspection it applies to an agency or instrumentality of governments which I think they use to include any corporation, LLC, or other state-licensed businesses (yes, they have used the word itself in the definition):
(c) Employee
For purposes of this chapter, the term “employee” includes an officer, employee, or elected official of the United States, a State, or any political subdivision thereof, or the District of Columbia, or any agency or instrumentality of any one or more of the foregoing. The term “employee” also includes an officer of a corporation.
http://www.law.cornell.edu/uscode/text/26/3401
I'm less familiar with the language for self-employment taxes, but that's contained within Title 26 as well (
http://www.law.cornell.edu/uscode/text/26/subtitle-A/chapter-2). Herein, the language states the tax will be "imposed", but doesn't say anything to the effect that the individual "must pay". Of course, one could look to Article I, Section 8 for the supposed authority to issue said tax, but this ignores the argument that such would constitute a "direct tax" not to mention the argument that the US Constitution doesn't currently have any authority over any living individuals anyway.
Anyway, it's all food for thought (discussion in the proper venues).
