I'll agree that American prisons aren't
as bad as the Chinese practices but it's a very long way from what would be seen as reasonable in much of the world.
When you say paid for example, the prisoners aren't paid statutory minimum wage, and don't have access to outside prices, so both how much they get paid (often less than a dollar per hour), and how much things cost them to buy with this money are both controlled by the same people controlling their labour, a recipe for abuse.
As to "optional" many convicts don't seem to agree with you about how "optional" their labour actually is. Convict labour makes the private owners a lot of money, so from their point of view a prisoner who won't work is a problem and problem prisoners get punished. US prisons routinely use harsh psychological measures such as long periods of solitary confinement that would be considered torture elsewhere - without meaningful oversight.
And finally "safety" is very much in the eye of the beholder. The relevant US agency simply doesn't collect any safety data about prisoners, reasoning that since work is part of the punishment for a crime of which they were duly convicted† it doesn't matter if they're injured or die as part of that punishment... after all the US still has capital punishment, deliberately killing prisoners is actually part of the "justice" system.
† Of course one of many other things wrong with the US criminal justice system is that some of these people never committed a crime, the lack of a sane pre-trial bail mechanism in most of the US means if you're facing prosecution you have to plead guilty to get out quicker if you're poor...