I take issue with describing it as "voluntary", when the alternative is a longer prison sentence. It is paid a pittance, and is effectively involuntary.
"Even though, in a lot of cases, it is technically voluntary, there can be serious consequences for people who refuse to work or who advocate for better working conditions because a lot of prisoners use working as a way of having their sentences reduced."
In that section of the article they are discussing labor internal to the prison, specifically maintenance and daily chores. Yes, it is true, while in prison you do not have a choice but to participate in that kind of labor and refusal will not help you be released early for "good behavior" or anything similar.
External labor, the kind discussed here - contracted by a 3rd party for money - is voluntary.
