From the IRS
A. The Voluntary Nature of the Federal Income Tax System
1. Contention: The filing of a tax return is voluntary.
Some assert that they are not required to file federal tax returns because the filing of a tax return is voluntary. Proponents point to the fact that the IRS itself tells taxpayers in the Form 1040 instruction book that the tax system is voluntary. Additionally, the Supreme Court’s opinion in Flora v. United States, 362 U.S. 145, 176 (1960), is often quoted for the proposition that "[o]ur system of taxation is based upon voluntary assessment and payment, not upon distraint."
The Law: The word “voluntary,” as used in Flora and in IRS publications, refers to our system of allowing taxpayers to determine the correct amount of tax and complete the appropriate returns, rather than have the government determine tax for them. The requirement to file an income tax return is not voluntary and is clearly set forth in sections 6011(a), 6012(a), et seq., and 6072(a). See also Treas. Reg. § 1.6011-1(a).