"above the law" doesn't seem right - the implication is that any prisoner can get time off for labor, much like a prisoner can get time off for good behavior.

The only difference is the kind of labor, but even then, there are probably prisoners with relatively cushy jobs - librarian, maybe, or office work. This guy was just allowed to do work that suited his skills.

If he had an advantage over the other inmates, it would be if he retains copyright and publication profits. Those probably stay with the prison or government.

You are right, of course.
Yet the matter does raise questions about why prisoners should be granted early release for working in prison. In the case of thieves, burglars, drug sellers, and other run-of-the-mill criminals, it makes sense to encourage them to establish good work habits, which would improve their chances of staying out of trouble upon release. But what about convicts whose offenses had nothing to do with their ability to make an honest living? For them, "work" in prison seems more a privilege, allowing them to continue their careers without significant interruption.