The reason the income tax is voluntary is that the so-called tax does not fit into the two great classifictions of taxation defined in our Constitution. If the income tax is a direct tax, it has to be apportioned. If the income tax is an indirect tax (how Bruashaber vs Union Pacific Railroad 240 US 1 made it constitutional), then it has to be apportioned*.
The easier way to understand about the income tax being voluntary is by using the fifth argument. That argument basically points out that when one files an income tax return, all that information can be used against you. One can use what a person puts on a tax return against one in a court of law (as Leona Hemsley found out). In such cases, the government regards Leona Hemsley as filing her tax return voluntary per one's Fifth amendment not to be a witness against oneself. Of course, the government also tries to throw people in jail for non filing so the government has been getting it both ways.
In the case you mentioned, Harry Reid admits that the government is illegally harassing people via our voluntary income tax and in another, Harry Reid openly violates the 501(c3) statute of the Internal Revenue Code to collude with Media Matters. We really have a criminal government -- especially today!
*for an explanation of the above, please see The Great Income Tax Hoax by Irwin Schiff.
I meant to say excise instead of apportioned as a direct tax is collected.
I regret the error.