I will claim no expertise on this subject but I think the "voluntary" part is filing a return. Paying taxes is not voluntary. When you "voluntarily" file a return you can claim all legal deductions to lower your tax liability. However, if you choose NOT to file a return then you pay the full tab as computed by the friendly folks at the IRS.
Fair enough. If it's mandatory (jail) then why do we have to execute voluntary withholding forms (W-4)?
It would seem that if the system was fixed then we wouldn't have a choice as to how we pay.
For example, your property tax assessment comes to you in the form of a bill. No voluntary compliance, just pay it or move.