The API being unauthd is clearly a core design choice, and finding out any customer or service data is openly accessible with consecutive numbers through that API is not a zero day or something.
There is no "responsible disclosure" to be made here, going to the company and explaining what's the issue with all of this amounts to "handing out free consulting" if anything
The poster here has no proof that the vulnerability was already being exploited. For all we know, as obvious as this was, no one else had yet thought to look.
This is like going around people's house doors, testing to see if they are unlocked, and if they are, posting a big sign saying "unlocked door" on each one. It's obviously an anti-social act masquerading as benevolent, and it should be punished. Of course, the company running such highly vulnerable code should also be punished, but that doesn't absolve anyone.
Noticing an overhead pipe at McDonald's is dripping onto the griddle and pointing it out to people isn't harming the business, it's pointing out the business' gross negligence.
If you want, a more apt comparison might be going around a business park and sticking big signs on every unlocked archive door you find. The companies not properly locking the doors are at fault, and customer data may already have leaked; but, you are virtually guaranteeing that even more customer data will leak by doing this. It should absolutely be illegal.
Secondly: can consumers be blamed for gross negligence? It's not reasonable for a bank to post account balances in public billboard and ask people not to look at others. We should contest when private data is available publically, hidden only by small obfuscations, not professional security practices.
Your example is flawed because in this case the private data was not made available publicly at all – you need to intentionally exploit a software flaw to access it.
Of course, it also matters how you handle it. If you do enough to just discover the flaw, try to adhere to the bug bounty program scope (if any), use your own accounts in testing and responsibly disclose any findings as soon as you have a poc then you'll probably be ok.
In this case the author went way beyond just finding the flaws, and then disclosed it publicly in a completely irresponsible way without even trying to contact the company or any of the clients affected by it (some of which will certainly have a security contact that can liaise with the vendor)
Maybe a better analogy is a bank with open lockers and no vigilance: if someone enters and steals money, the police will look for them, because "the coffers were open" is not a valid defense. But customers will also demand answers from the bank - why were they so negligent and incompetent that someone can just enter and get their money?
We should hold similar values for digital systems.
Was the author's intent on stealing private data and causing harm? Did he gain from this abuse? Did the company take enough measures to safeguard their data?
Companies have been mostly not held responsible for their fuck ups, and no matter the law, that's wrong to me.