The story goes on to reveal a hidden danger emanating from the Zone, but I want to focus more on Zor-El’s comments, because I think they offer an interesting argument on punishment and personhood itself. (I will ignore the fact that the physics of the Phantom Zone make no sense—it is said that time stands still, and that criminals have eternity to reflect on their crimes. But any action or thought takes place in time, which therefore cannot stand still--what they really mean is that the Zone criminals do not age. Of course, the dimension that the four Crisis on Infinite Earth survivors were in until Infinite Crisis had the same problem.)
Jor-El and I had endless debates on the ethics of his "Phantom Zone." I found
the moral implications of his ‘compassionate punishment’ distasteful.
People weigh heavily the cost of condemning a man to death… but a "living death" in an "endless void?" How easy would it become to simply erase our "undesirables" with the press of a button, freed from consequences?”
Zor-El is really making several arguments here: one is that exile to the Phantom Zone will make punishment too easy for the Kryptonian authorities, and they will be too quick to use it if they are under the impression that there are no drawbacks to its use. This is more of a political argument about the possible abuse of state power, and is a perfectly valid issue.
But I find his second argument more interesting—is eternal imprisonment in an “endless void” truly more ethical than execution? Those who feel that execution is always wrong may believe so simply by default, but they have to show that Zone exile is less wrong. One such argument could be that however horrible it may be, prisoners can be released from the Zone, making the sentence partially reversible. (This is an especially cogent argument in light of recent reversals of death penalties as newly discovered evidence of innocence comes to light, which provides an argument against capital punishment even for those who believe that murderers deserve to be executed.)
This relates to the justification of punishment itself: do we punish to deter future crime, or to hold criminals accountable for (past) crimes? In other words, do we punish because it's good (through deterrence), or because it's right (by retribution)? If Phantom Zone imprisonment is indeed "cruel and unusual," then this raises more of an issue for its being right than being good - after all, cruel punishment is likely to be more deterrent, unless such a practice is so abhorrent that it incites rebellion against the government which practices it. (This is the standard utilitarian argument against punishing the innocent.)
But the more essential issue is not even a legal or jurisprudential one - it is one of personhood. What does it mean to be a Phantom Zone inhabitant? What is the nature of this "void"? Is individual identity, agency, or rationality preserved? This was Zor-El's more humanitarian concern - what are doing to these people? Even strong retributivists like Kant forbade torture because it violates the dignity of persons (even those committed of heinous crimes), while they often recommended capital punishment.
Is Phantom Zone imprisonment akin to torture? Is it a more humane alternative to execution, or is perpetual exile from anything we think of as reality even worse? This are important questions, and an unexplored facet of the Phantom Zone (at least as far as I know.)