So I'd argue that it wasn't GitHubs backyard. They might be the landlord but they can't take ownership of the things that you build in your (rented?) flat.
Don't get me wrong, I also agree with you. For me this whole situation is kind of a paradox where there's no easy moral (or legal) answer on what to do, and on what society already has agreed upon.
Completely irrelevant to the issue at hand, though. The copied code was copied in accordance with an open source license. By this same token, the affected users/companies are free to start their own fork, but they didn't. The developer shouldn't be under any obligation to maintain anything, and GitHub shouldn't be intervening in these kinds of situations as that will simply serve to dull the positive effects these scenarios could have on the dependency landscape (people actually figuring out their shit). This is the package equivalent of a bail-out. At the end of the day it hurts more than it helps.
I see it like this. I can't buy a Harry Potter book, translate it to a different language, maybe change some of the character names and then claim ownership while not acknowledging the original work even if I publish it for free.
It's like you and your friends building a Mustang in your backyard, all of you put time, energy, and even money into the project only for you to turn around and claim you invented a new car model built entirely by yourself. No, it's a ford and most of the upgrades were done by others. People that helped you might feel that you are in fact being malicious.
Since we're using analogies, this wasn't his backyard, this is the GitHub's yard.