Sounds incredibly dangerous on a lot of levels. Prison labor is just slave labor, pure and simple.
Is it slave labor if they're volunteering? And how many ways dangerous? They're generally given very basic jobs that assist with fire prevention rather than fighting active fires.
I think so, because it's not really a choice. If you're rotting away in prison, you'd do almost anything to get out. Prisoners (depending on the state) are often charged outrageous fees for tiny conveniences like phone calls or clothes. The state of Ohio is actually charging prisoners for their entire stay and then demanding they pay for it or go back to jail when they get out. Under an oppressive system such as that, prisoners may have no choice so they can get out from under their debt and then the pay is just shit, so they don't.
And it's dangerous, in that, firefighting is always dangerous. As this program continues, the prisoners may get expanded roles and more duties. They maybe eventually be offered more time off for riskier duty. On the other side, they are criminals doing work near an area where there's a lot of smoke, confusion and danger. Escaping may endanger the lives of everyone around them and/or they may get themselves and others killed in the attempt. And although the article states that the sort of criminals being used aren't the hardened types, some criminals merely are not hardened on the record. They may have gotten away with scores of crimes authorities know nothing about and then use the fire to either escape, endanger people or even kill people.