Re: "Yet this mobile app is not open source"
Now it begins? The GitHub website has never been open-source, so there's no rational reason for anyone to expect them to make the apps open-source either.
"Open source has won" is a great sentiment espoused by many companies (eg. GitHub, Microsoft more generally, Google) who continue to make the bulk of their money from closed-source proprietary software.
Biting the hand that feeds IT
