I take some issue with the word "leak" being used to describe this.
What Google discovered was a vulnerability. And they discovered it internally; it wasn't reported to them by an external party. If someone else had discovered this vulnerability, they might have been able to exploit it to gain access to user data, and then it would have been a leak. But there's no actual indication that this happened. Google can't say with absolute certainty that it didn't happen, but so far all signs are pointing toward this vulnerability not having been exploited.
Obviously if there had been an actual leak, then Google would have a responsibility (both legally and ethically) to disclose it. But if we start requiring software developers to disclose every bug that could potentially have led to a data leak, then we start getting into some real slippery slope territory.
All software has bugs. Some bugs are small and some are large, but there will always be bugs, and sometimes those bugs will be in code related to security or privacy. Chances are very good that most of the software you use on a daily basis has more than one security bug that could lead to a data leak. But chances are also very good that no one knows these bugs exist.
Eventually, someone may discover them. The best case scenario is that the person who discovers these bugs is the developer, and they fix them before anyone else knows about them. That's what seems to have happened with Google+.
But if we require developers to disclose any bug they discover internally that could have any implications for data security, that creates a strong incentive against trying to proactively identify security bugs internally. And it discourages companies from hiring security firms to perform audits, because if those audits reveal bugs (and they usually do), the obligation to disclose them could result in significant negative PR. So the safest thing to do financially becomes to avoid looking for bugs and just hope nobody else finds them either.
I really hope we don't end up in this situation, because we would all suffer.