It shouldn't also take away from the rights of others. When prison labor is obtained below minimum wage, or under conditions or terms that workers would not accept, you hurt low income workers and drive down their wages in aggregate.

In my opinion, prison labor should only be used for public works. Cleaning up parks and that sort of thing. Giving a captive labor stream to private enterprise is only good for the owners of the select few firms who can take advantage of it.

I agree with the sentiment but it should be pointed out that even public works have to pay their workers at market rates when there are no forced laborers (so: most of the time). Throwing a bunch of forced laborers at public works kind of screws over the average Joe that used to do that for pay.

So, like you say, it shouldn't take away from the rights of others. Just pay the prisoners just like you'd pay any other worker regardless of the kind of job.