This is from a 1999 interview with Christian Parenti. A much fuller version of this argument is in his book "Lockdown America"
[WS:] And this is a 2002 paper by Yanling Dai from China's Ministry of Justice: http://www.iuscrim.mpg.de/forsch/onlinepub/chinese_prisons.pdf
He argues that "the Chinese prison system uses prisoners labour as a fundamental way to reform criminals. It is an ideal way, by forcing criminals to work, to encourage them to cultivate the habits of hard works and thriftiness by their effort and master some labour techniques which will prepare them for their re-entry to society."
AFIK, all socialist countries used prison labor in a similar way.
A similar idea was one of the central themes in Bertolucci's film "The Last Emperor." It shows how the last emperor of China was transformed by the prison system using prisoner labor, from an idler, drug addict and a traitor to a productive member of society. I think the scene in which the ex-emperor, being asked by a Red Guard during the Cultural Revolution 'Who are you?" replies "I am a gardener" (a skill that he learned in prison) is one of the most profound in cinematography. Bertolucci has good leftist credentials, and by no means cannot be accused of harboring conservative "law and order" sentiments.
In sum, the idea of rehabilitation of criminals through labor has good left-wing credentials, although perhaps not in the US - which does not surprise me, since this country started, in part, as a penal colony.
Wojtek