Yes they are voluntary but they must be paid up front.
V4V they are not.

You pay up front with no knowledge of the quality of the content you are paying for as it is not delivered until after you have paid.

V4V is interactive- you like some content and send zaps to show your appreciation of it.
A subscription is not interactive.

You could pay up front then get complete crap of zero value for that payment.

V4V puts the onus on the content providers to provide quality upfront.
Without that onus it is not a V4V proposition.