The labor performed by prisoners of the US prison system isn't comparable, you're right. But given that the US imprisons at a vastly higher rate than the rest of the world, and:
Over all, there are now more people under “correctional supervision” in America—more than six million—than were in the Gulag Archipelago under Stalin at its height. [1]
That, I think, is a useful comparison, and one that we should be making and asking questions about.
We can play "Six Degrees Of Kevin Bacon" with this and connect injustices in the US to virtually any story through virtually any starting point. For instance, you could just as convincingly have argued:
"But given that economic inequality in the US is so high, and given the demographics of the kinds of people sent to prisons, we should be asking questions about the parallels between Soviet slave labor and US prisons."
So you argue (at the risk of putting words in your mouth: "you can't compare USA and USSR prison camp systems because the work conditions are hugely different".
Then I respond: "but the systems are similar in the way they both grabbed unprecedented levels of citizens, and possibly the justice of their convictions".
Why are the dots you've decided to connect the right ones, while the dots that I think are important are just playing a game?
Although the Gulag system was intended to be an engine of productivity in the USSR, that goal was never achieved. Throughout its history, the work camps were never profitable, they never carried their own weight. So it's hard to say that in the USSR, the work camps were benefiting the population at large.
Over all, there are now more people under “correctional supervision” in America—more than six million—than were in the Gulag Archipelago under Stalin at its height. [1]
That, I think, is a useful comparison, and one that we should be making and asking questions about.
[1] http://www.newyorker.com/magazine/2012/01/30/the-caging-of-a...