OK, there's definitely some disconnect here between what I think I'm arguing and what you seem to be arguing.
For starters I can't see what your idea of punishment actually is, since you say it's good practice to punish unfairly/fairness doesn't enter into it. That makes no sense to me. The entire point of punishment is to reinforce rules - if there is no rule involved then how can punishment be anything but arbitrary? And how can arbitrary punishment ever be useful? How can anyone learn from that?
Even if a child does do something bad that it couldn't have known beforehand was bad - how does it help to punish them instead of simply reprimanding them and letting them know this is an act that will merit punishment if done again? I can't think of an example where this would be helpful.
The issue of whether he deserved it or not is nonsensical. If I had spared him the terrible experience, he would have walked around with feces on him, getting himself and everyone in the house sick.
That's the nature of punishment to a good parent, it isn't a matter of whether the punishment is fair or deserved or forewarned. If you have the knowledge that putting your child through a minor unpleasantness now will spare them far worse later, that is the expression of love. Talking about fairness in punishment, or assuming that any punishment is equivalent to taking out your anger with a belt is missing the point entirely.
But what you describe with the cat is not punishment. The mere fact an experience is unpleasant or unwanted, even if it is a consequence of a chosen action, does not make it punishment.
Washing a dirty cat is not punishment. Swatting a cat's butt when he's chewing on the houseplant is punishment. The cat can learn nothing from the first - he's not even supposed to. The cat can learn something from the second. (then he will ignore it

) Even my friend's idiot cat, who likes to crap on the carpet, knows he'll get punished for it. He squats and then runs and hides.
Yes, it is possible for something to be perceived as punishment even though it is not, but to me that is a different issue. To extend that to a metaphor explaining either the problem of evil or the dissonance of a loving god that sends people to hell forever... Well I can maybe buy that natural disasters are just things that need to happen and are not punishment (though some loud evangelists have characterized them as punishment), but I fail to see how anyone can twist the entire traditional idea and presentation of hell to the point it's not actually punishment.
To reiterate, the ENTIRE idea I'm taking issue with is the fundie view of a God hellbent on punishing, in the sense of 'don't do that or I'll smite you', anyone who isn't in the right club doing the right things, and the idea that the brimstone God they describe behaving this way is also perfect & loving. This God doesn't get the 'cannot communicate, sorry' caveat; this God's fans claim to be in communication with their God all the time.
It isn't mutually exclusive for an entity to create the universe, have the final say in our afterlife, and still be unable to communicate it's will directly to humans.
That's fine. In which case, though, that entity has no justification for punishing anyone for breaking rules they had no way to even infer. Again I do not call natural consequences punishment. Punishment very specifically is an intentionally imposed penalty for wrongdoing.