That is true about unpaid labour, but my understanding was that this isn't forced on to the prisoners. I understand that prisoners are given the option to work, and reap various benefits in return (modest pay, extra hours outdoors in the case of road crews, and potentially leniancy for "good behavior" at parole hearings).
And yes, this has absolutely been interpreted in this way. Unpaid, or grossly underpaid prison labour is extremely common in the US.