What if a particular piece of code is licensed restrictively, and then (assuming without malice) accidentally included in a piece of software with a permissive license?
What if a particular piece of code is licensed permissively (in a way that allows relicensing, for example), but then included in a software package with a more restrictive licence. How could you tell if the original code is licensed permissively or not?
At what point do Github have to become absolute arbiters of the original authorship of the code in order to determine who is authorised to issue licenses for the code? How would they do so? How could you prove ownership to Github? What consequences could there be if you were unable to prove ownership?
That's before we even get to more nuanced ethical questions like a human learning to code will inevitably learn from reading code, even if the code they read is not permissively licensed. Why then, would an AI learning to code not be allowed to do the same?