What if you can't send them back? Keep in mind that in the "present day" Trek Lit world of approximately 2385ish or so, punishment is on a pretty nice penal colony. Gardens, exercise, fresh air, education and rehabilitation. If we punish past criminals according to "current" law, it will seem like an amazing vacation to them with much better living conditions than they were used to-for the most part. I think they should be prosecuted according to modern day law, so we don't unleash someone with a murderous/criminal past onto Federation society.
Except that along with that more benevolent penal system is a far more sophisticated system for rehabilitating criminals. That's what prison is supposed to do, after all -- not just lock people away for a while, not "punish" them by subjecting them to more trauma which will make them even more dangerous and antisocial when they get out, but reform them so they won't be criminals anymore. The American penal system these days has forgotten that basic principle, but clearly the Federation has remembered it and made it work, because they've successfully banished virtually all crime.
Besides, aren't you conflating two separate parts of the process, prosecution and sentencing?