That is forced work on an imprisoned person upon threat of punishment, even if they can theoretically decline it.

There is a history of this yes, but there is no signs that this is happening with this specific situation. I even said, if this case had that, it would be slavery.

The website for the program, while can’t be fully trusted, explicitly states that this is not the case

An incarcerated person must volunteer for the Conservation (Fire) Camp Program and meet all eligibility criteria meant to protect public safety. No one is involuntarily assigned to work in a fire camp. Thus, incarcerated people do not face disciplinary action if they choose not to serve their time in a fire camp.

Edit: And just to be clear - Yes, they might be forced to do something else if not this, but that’s probably up to the prison specifically. That alternative would be slavery.