"Why is it reasonable to believe in prisons, but not in the hell?"

[The Grand Nudger]

An invalid argument against the existence of hell is an incredibly poor counter argument to the claim that for there to be justice in the world, an afterlife has to exist. That if all those evil people don't get punished in the afterlife, when do they get punished?

The second premise of that argument is question begging so we can discard it for the time being. If we wanted to argue the first, we would be arguing against the premise that hell is necessary to justice. To that end....the contention that hell itself is unjust seems compelling but is technically not relevant.

The claim that hell is necessary to justice is not equal to the claim that hell is just.

If, however, we focus just on the absurd claims - your notion of a prison(or hell)'s nonexistence via injustice and the faithful's question about when or where evil is punished if there's no hell can both be addressed with a functionally identical response. Bad Things™ and Bad People™ exist, none of them flit out of existence on account of their badness and if that badness isn't addressed before they -do- flit out of existence, in either case, then evil doesn't get punished. The idea that the world or some aspect of reality has to be that way because it should be that way - with reference to whatever a persons position may be. It's a variant of the naturalistic fallacy called the moralistic fallacy. Briefly, the idea that something is right because it's natural, or natural because it's right. Or the converse.

That the societal unpleasantness of a thing or state of affairs implies that this thing cannot exist. You refer to the unpleasantness of prisons, they refer to the unpleasantness of unpunished transgression.

So, btw, is the believers first premise and the silent assertion it belies - the claim that hell is necessary to justice is a claim about the nature of justice. That for justice to be instantiated in nature and by it's nature there should be such a place, therefore there is or must be such a place - if there is justice. We can point out that should be and is have a poor track record in the switch from what is natural to a thing or natural to a state of affairs described and it's moral import. Or, we could simply accept the flawed premise and conclude that..if true..then justice doesn't exist.

If justice in this world truly relied on a superstition, there's no other conclusion to be had. There's probably a whole lot more interesting stuff going on in a moralizing believers head than this little bit of wrote performance. Hell isn't a rational product - but their underlying moral contentions may be...and it may be that the rationality of those products lends it's weight in credence and credibility to those other propositions that swirl around items of local superstition.

Have you ever considered whether you believe that people should get more, exactly as much as, or less than they deserve? Does your opinion on any of those three items change when we change the thing done from good to bad? Do the good deserve more praise than their actions merit, exactly as much, or less? Do the evil deserve less punishment than their actions merit, exactly as much as, more than? Our answers to these questions as individuals are likely to shed a hell of alot of light on whether and why a given person believes in hell. The camp that posits hell is at the two poles of that axis, where evil deserves extreme punishment and human good is as mistaken and prideful a folly as praising it. That only fealty to a blood god will allow you to completely abdicate your moral responsibilities and the insisted consequences of evil, evading both poles, in a final and complete fuck you to any coherent notion of justice within or without the articles of the superstition in question. It's relatively easy to predict a persons responses about hell (and heaven) based on genuinely held answers to the question of relative desert.