> No, it isn’t. If the project is under an open source license, it means that everyone can share a copy – even on GitHub – of the licensed material under certain conditions. A license restricting this right wouldn’t be open source anymore. However, since GitHub may not respect the terms of licensed code that is hosted on their servers, not uploading the code of others there is, in fact, an ethical choice.
emphasis mine. It's a "please be nice", not a "I want to enforce things"
> Is this a legal document?
> No, it isn’t. If the project is under an open source license, it means that everyone can share a copy – even on GitHub – of the licensed material under certain conditions. A license restricting this right wouldn’t be open source anymore. However, since GitHub may not respect the terms of licensed code that is hosted on their servers, not uploading the code of others there is, in fact, an ethical choice.
emphasis mine. It's a "please be nice", not a "I want to enforce things"