Federal, sure, but not all inmate firefighters are in federal prison, and some (most?) states have voted to remove the 'except as punishment for crime' bit.
Removing the language doesn't suddenly not make it slavery.
When the states remove that language it means prison labor programs (such as the one in California) are unconstitutional. Why? Because those programs are slavery and when that language is removed those programs no longer have the exemption that made it constitutional.
Wat? Are you arguing that all imprisonment is 'slavery' since it entails physically constraining prisoners? With that definition any institution that 'physically constrains' people is slavery, including public schools which prohibit students from leaving during school hours. Those students are physically constrained from leaving the premises, for example. In many cases there's even a cop to enforce it.
You mentioned Colorado, and I implied Colorado and others that have followed suit, in my earlier comment which I assumed that you read before you wrote this reply.
Your #1 source does not mention the text you attributed to it. And it's an opinion piece. It mentions that the inmates who volunteer to work do so for low wages. Well, they could also choose not to work on those programs. What proof do you have that they were coerced (I'm actually not arguing against it, but looking to correct my opinion if necessary).
In any case, states do things that the federal government does not agree with all the time (legal weed, for one). It's a crucial feature of our system of government.
That's not at all what I'm saying. The important distinction you're missing is any labor that is performed without fair compensation (in the U.S., that would be minimum wage) while physically constrained. Mandatory schooling as well as the draft have been adjudicated under the Thirteenth Amendment and are not within the definition of slavery. Those cases helped shape the definition I provided you.
You also don't appear to understand the consequences of Colorado's Constitutional Amendment. By removing those words from the Colorado Constitution the "volunteer" prison labor programs are no longer permitted. These programs are, legally, slavery. They just so happen to be a legal form of slavery under the Federal Constitution because of those important words "except as punishment for a crime." When those words are removed from the Colorado Constitution that exception no longer exists for Colorado prison operators (because a State Constitution cannot contradict or limit an individual's rights under the Federal Constitution but it can provide individuals with additional rights). Because these "volunteer" programs are slavery, and because Colorado's Constitution now prohibits all slavery, no exceptions, these "volunteer" programs cannot operate.
Prisoners can't freely volunteer because their liberty is at stake. They get to choose between their life being in danger in prison or endangering their lives fighting forest fires and perhaps getting out of prison a bit earlier. This is evidenced by the very low wages. Would you go fight these forest fires in California for $1/hour? I highly doubt it. Why do they "volunteer" then? Because they want their freedom. Think about the lengths you would go to in order to regain your freedom and then think about how much autonomy you would have in arriving at your decision. It's Sophie's Choice.