Seems to me like he's bringing this unethical decision to attention so that employers and employees know that it's not okay. I disagree with you. It's not petty, and certainly not childish.
Maybe not, but it's definitely the action of someone lacking experience. Anyone who has been around the block knows that companies can be very petty. Just because a work environment is 'friendly' and 'family-like' doesn't mean you can count on that kind of behaviour when you exit. Companies turn petty very quickly. Very quickly.
An experienced person would have made sure all the money he/she had coming was collected before even handing in the resignation letter. Once you've signalled that you are leaving, things can go south very fast.
I just look at this whole situation and what the OP did and think "what a rookie".
It serves to inform potential employees of what they should pay attention to when negotiating with this company. Is it that it puts some tiny bit of negotiating power into the hands of potential employees that makes it distasteful for you?
It's always better to say positive things in public than to post something like this. However, it also sounds petty and childish on Miso's behalf. They are the ones who hired the author, and they're the ones who set the stage for what constitutes appropriate behaviour, so the whole episode just looks ... bad.
