legally perpetuating slavery?

I'm re-reading the Autobiography Of Assata Shakur at the moment (the first time was for an African American Literature class and I skimmed that shit just enough to be ready for an exam) and Assata makes an interesting point. If you take a look at the 13th amendment to the constitution, you'll find that it says:

'Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.'

Needless to say, I was pretty shocked to find out that slavery is, indeed, still legal in our country... it's just been shifted to the prison system. After reading this amendment, I couldn't help but notice how 'coincidental' it seems that prisons are disproportionately populated with black and 'third world peoples' as Assata puts it. If every state had to pay workers to do the jobs that prisoners are pretty much forced to do for 'slave wages', the salaries would amount to billions.
When Jimmy Carter was the governor of Georgia, 'he brought a black woman from prison to clean the statehouse and babysit for Amy'--- I remember learning that this was Alice Walker's inspiration for the character of Sofia in The Color Purple, by the way. Anyhow, Assata Shakur goes on to say that prisons are a way of legally perpetuating slavery and genocide and that new prisons certainly aren't being built with the idea that they will be populated by white people.

My take on it... to be honest, I'm still a bit ambivalent about the whole thing but it sure is growing increasingly hard for me to just chalk it all up to 'coincidence'.