What part of voluntarily giving up their wealth defies the definition of "voluntary?"
Is the place where the word in contention here. Just like if I point a gun at your hand and tell you that receiving the bullet is "voluntary" because you can always voluntarily cut off your hand.
That being said, I agree with the concept of taxes, but let's not kid ourselves and call them voluntary.
You could make up a reason, but that's stretching even your definition of "voluntary" under the law.