The IRS has been calling their system of tax collection "voluntary" for decades. Using that word in a way that is clearly different from its normal use is a part of their attempt to deceive and gloss over the violence inherent in their system.
Evidently, according to our friendly government folks, if you do it after only a threat of force then it's voluntary. On the other hand, if you or I threaten force and a person responds it is not voluntary, it is coerced and considered an assault.
Personally, I always consider tax preparation and filing and payment to be done under duress and the threat of force. Consider what the US Federal govt does with the money I "voluntarily" "give", I feel I would be supporting theft and murder around the world if I did it any other way. If it was legal to withdraw my support from the system without being independently wealthy, I would. And no, I don't think going on welfare would constitute withdrawing support from the system (though it might help speed its demise).