Opponents are right in the same way you shouldn't badmouth former employers in job interviewers. It's not fair but it makes you look worse. You just kind of have to suck it up and move on.
When companies screw you out of money, you should name and shame them (IMHO). Trust me, over the years I've probably lost out on all told $50,000 or more that people never paid. In some cases it was a clear breach of contract. I never pursued it through litigation and reading stories from those that have I'm kinda glad. Such a thing is a mental burden and a distraction. Your best bet really is to move on unless the amount is really huge (maybe even $50,000+ for a single claim).
All that being said, this scenario is not one the company should be ashamed of IMHO and were I Miso I wouldn't have said a damn thing about it.
He forgot to claim a referral bonus. They didn't pay it after he left the company. Technically, they are correct: referral bonuses are paid to employees. He is not an employee.
Sure they could've paid it to him and maybe they should've but I sympathize with their position.
No matter what anyone tells you--and this is important--when you get a bonus of any sort it is forward-looking even though it's for what you've done. If you've left the company or are leaving, you are no longer of value to them.
If he's made the claim and pushed back on it finally reneging, that would be a completely different story.
This is a textbook example of "nothing to gain" and "makes you look bad" rolled into one.
EDIT: to answer one point brought up by a commentor (chasing): yes, you are correct. This is not a bonus in the strictest sense.
It is not however automatic compensation like your salary is. You need to take action to claim it (by submitting the form or whatever to get the referral bonus). Miso admits he did not do that. He is no longer an employee and thus can't file forms an employee otherwise would.
I imagine if this were a case where he expensed something and failed to claim the expenses the employer would pay. From this you can (reasonably) view the two as similar. The difference (IMHO) is that if an employer doesn't pay outstanding expenses, you may continue to own or you have a pretty cut-and-dried case against them for breach of contract.
Failing to file a referral bonus form while you were an employee is really your fault.
I'm not saying the employer is completely in the right here but honestly I think they're more right than he is and certainly come out of this looking better than he does.