This is... probably misalignment with modern definitions. Volunteer for the time was not at all the same as what we think of today. To wit, many of the "volunteers" were as likely getting the only form of compensation that they could get by being there to build something.

Specifically, if there was a local market, it would have been near the cathedral. And if you didn't have a reliable farm or other plot of land to provide food, seems reasonable to assume your best bet was to be compensated in some form for manual labor.