The one thing being using convict labor. It starts getting a little ambiguous once you start thinking about the second generation. Or even release terms on the first-gen group.
I mean, what good is parole or end-of-sentence release if your bones are wrecked from ten years in low-gee? Is the gub'mint gonna pay to rehab a con, or are they just going to leave them to rot? Wouldn't that be considered a sentence of exile without a verdict in court?
What about families? Are you going to transport a con's kin with him? What about housing and food? Schooling for the kids? Their rehab to enable them to go back to Earth?
I'm guessing a rebuttal might go something like this, "Use lifers or death-sentenced cons! And their families have to stay groundside!" In that case, what motivation can you offer them? Why should they work if there's no chance for clemency or a reduced sentence or even a visit from their mom? And in many cases, are they even able to do the kind of work expected of them? I haven't heard of many astronauts or aerospace engineers being convicted lately.
Also, what about guards? Are they to be exiled as well?