First off, if this were a good argument GitHub might have made it, but they haven't, have they? They're claiming fair use instead. If they want to make the argu

First off, if this were a good argument GitHub might have made it, but they haven't, have they? They're claiming fair use instead. If they want to make the argument it's covered by their TOS then we can consider it. Secondly, if I release my code under a license that doesn't permit this sort of thing, and someone else publishes the code on GitHub, I haven't agreed to GitHub's terms and the person who did can't give GitHub any more rights over it than I gave them. Not every piece of code on GitHub was written by people who agreed to the TOS, it's quite easy for open source code to move around. Suppose the maintainer of a project moves it from GitLab or whatever, with your contributions intact. The maintainer only has your code because you contributed it under the AGPL or whatever- they can't "license" it to GitHub under more lenient terms than that!