I can give a dog walker or cleaning personel the keys to my apartment, still if they steal stuff and I have evidence they will be prosecuted. It's not a bug that they don't have business ethics.
BTW, just in case you are unaware, Equifax got away with this hack with zero fines in US.
Most of the other Facebook data breaches where they didn't secure data accordingly would compare more to what you refer to.
This case is different though as Facebook performed unauthorized actions on email accounts, basically breaking in.
A bug is a bug. Whether it allows a hacker to sneak in to steal all your data or whether it allows a company to collect data it wasn't supposed to (as in this case Facebook specifically mentioned that it didn't turn off the feature though it intended to).
What you are describing here is in fact a lack of action, or a lack of change policy (to cause such action). That's not a bug. A bug is unintentional behaviour of some code, not some folk who've said they'll do something, but then don't.
And as for whether the original behaviour is/was a bug is also a point of contention too: that's a lot of willfully bad behaviour that's got chained together somehow to do what it did, then reviewed, signed off, and deployed — that's quite some 'accident' — I write code, and to me this whole thing just smells of a cover-up (by FB calling this a 'bug', when it very much looks to be otherwise).
I doubt it, so it seems that we're just bickering over whether the accidental removal of the message is considered a "bug" or a malicious act by some engineer to trick users into sharing their data because they (and their company) lack business ethics.
Which is more likely?
Maybe a complete engineering stop for a few months, and development of new practices and processes.
Similar to what Microsoft did with Bill Gates Trustworthy Computing memo which led to the creation of the Secure Development Lifecycle is something Zuckerberg should order to do.