'Murica
The problem is that most employment contracts in IT contain some variation of "Any IP/patentable thing you make while working here is ours", with some extra language that basically suggests anything to do with a microprocessor is "proprietary" or a "trade secret", thus developing it on your own time still means they can steal it because you used the aforementioned "proprietary/trade secret" knowledge learned on the job. And because the judges in this country were all elected from the brain slug planet, they generally run with that. It takes a very expensive legal battle, with many lawyers, and many appeals, to get some semblance of sanity out of the system.
In America, and to a lesser extent western society in general, someone has to own *everything*, and it slants heavily towards corporations owning it all because there's the perception that is better for society than letting an individual keep it. It's also how all music, software, and other copyrightable goo had language snuck in by a congressional scribe (ie, it was never voted on, and was done by someone whose job description is to correct spelling errors and the like) that added a whole clause making all of it a 'work for hire' -- meaning if you get money for it, it's owned by the person who purchased it, not you. Sell it for anything at all, and you lose all your rights as an artist.
This is about as useful as those people that tried to send modified http requests with embedded cookies saying something to the effect of "By continuing this connection, you agree to my terms, which supercede your terms." Needless to say, it had zero legal weight. Sorry, but this move by github means absolutely nothing unless they're willing to setup a straw man case and get a precident set that it does, indeed, supercede other contracts without exception.
And I seriously doubt Github is going to blow hundreds of millions of dollars (or more) to try to overturn decades of corporate rape of creative works. Remember, our court system entertained a multi-year hundred-plus million dollar legal battle with several suits... over whether or not beveled edges on a cell phone was a valid patent. The courts reversed repeatedly as it went up the appeals chain. There have been similar examples of exceptional absurdity in our courts. The open source community should well remember SCO and it's unending battle with the entire universe (IBM principally, but they've gone after many organizations) which has been going on for over a decade in various forms, and the courts still keep entertaining it.
It would be cheaper, and more efficient, to simply round up the legislators, business leaders, lawyers, and judges responsible for the mess and dump them into a volcano. Or a shark pit... i'm flexible. I don't know there's enough sharks in the ocean, however, so volcano currently tops my list.