I always got the impression the federation's penal system was pretty lax and comfortable.
Well, I'd say "pretty lax and comfortable" for the prisoners who line up without discussing the settlement and work! I'd add that, as enchanting as the landscapes are and even if the work didn't seem too stressful (by the way, nothing says that Tom didn't take a break to cobble an old device when Janeway arrived unexpectedly...), a prison remains a prison, full of people (prisoners, guards) not always well intentioned towards you ; with a well-defined regulations from morning to night ; rationed food (and not the best one) and no alcohol
etc... . Please, do not be fooled by appearances, mon ami! ;-)
-> I guess that those who are there, know the difference between their previous lives when they were free to do what was going through their heads, their present life where activities are restricted and the one they would like to have when they come out from there, if they are quite lucky.. For Tom Paris, I'd say, plenty women/alcohol and enough money to cobble old devices/write holoprograms! :-D