There are interesting links to slavery and workhouses by allowing people to do actual labor from prison.
There is pretty much no way to allow people to 'volunteer' to do this kind of work without giving someone an incentive to get as many people into prison and using it as a giant slave work camp.
It's an open source project, that he originally contributed. It's a far cry from "slave work" if the work is voluntary and the result ends up in the digital commons.
In Greece you can optionally work while incarcerated, provided there is work to do for some of the prison's facilities, and each workday counts as two regarding your stay there.
For the same reason why prisons don't allow any other outside activity, whether commercial or not. This would require pretty big prison reform. And that is assuming he has any interest at all doing that.
Hans Reiser was reportedly impossible to work with in any event, as the Reiser4 debacle showed. I'm happy excluding him from the open source community.
There is pretty much no way to allow people to 'volunteer' to do this kind of work without giving someone an incentive to get as many people into prison and using it as a giant slave work camp.