They used words like patriotic duty when world war one and two hit as a way of shaming those who might say they should not pay taxes on labor. Of course people were willing to be patriotic.
And there is a real reason why your 10-40 says voluntarily! Because if it said you have to pay the tax, it would be unconstitutional. What is also strange is if you have to pay the taxes and legally, why does the government not provide an office ect. that helps you do your taxes? Any other outfit like insurance companies etc. tell you how much you owe after copay bla, bla, bla, bla....... but for an agency to tell you to tell them how much you owe...? And they love to get over paid as well. Come on, there is more to it boys and girls. Hmm, maybe its illegal, but once you sign your name, you agree that you owe, and if you lied about any of it, you committed fraud and can go to prison.