I believe that the concept of "voluntary" is quite abused by Libertarians and especially the Austrian school in not recognizing the inherent power differentials and coercion that creep into any relationship where one is selling one's labor to another.
Austrian economics is only descriptive and with some capability of prediction, like any other science its amoral. Recognizing the morality or inmorality of some arrengement is not the task of austrian economics, the same way recognizing the morality or inmorality of an arrengement is not the task of physics or medicine. Science should describe an arrangement and predit its results. The task of saying if it is moral or inmoral belongs to morality/politics/philosophy.