The Social Security "contribution" from wages received by an employee was really not a savings plan, but rather, in the Supreme Court's words, "a special income tax . . . measured by wages:"
http://en.wikipedia.org/wiki/Helvering_v._Davis
The words "measured by" are a tip-off that a government privilege is being extended because those were the same words used to describe the corporate income tax in Flint v. Stone Tracy (1911), where the extent of the government-bestowed corporate privilege is "measured by" the corporation's net income:
http://en.wikipedia.org/wiki/Flint_v._Stone_Tracy_Co.
The Social Security tax is two part. 1. A tax on employers for having employees (employer/employee relationship) 2. A "Special income tax" on employees as measured by wages. Half the tax is on the employer and the other half on the employee.
So what makes it so "special"? The court did not say...but the "measured by" part gives a clue.
Also in A.L.A. Schechter Poultry Corp. v. U.S., 295 U.S. 495 (1935) Justice Roberts statements essentially lay out the path to government intrusion. "We have held in . . . Schechter Poultry . . . that Congress has no power to regulate wages and hours of labor in a local business. If the [government] is right [in Butler], this very end may be accomplished by appropriating money to be paid to employers from the federal treasury under contracts whereby they agree to comply with certain standards fixed by federal law or by contract."
Using money substitutes other then those emitted directly by the Treasury under their exclusive money powers could certainly in my view be a taxable excised activity.