One can argue about ToS and copyright, about different interpretations of fair use, derivative work, DRM protections, and so on. Usually people are not interested to discuss finer details of those things. Most people seems to want to perceive it as either being public or not public, in which case, Youtube is just as public as PyPi.
It's a fallback clause, "fair use" is irrelevant if you've already given github permission to use it. By adding that clause you can no longer argue that it's not fair use to use the code you put on github after agreeing to their terms.
You can’t use a license to bind people who don't need a license.
Your reciprocity suggestion could also work, since it would mean any LLM trained on even a single file of GPL code would be "poisoned" in the sense that any code it produced would also be GPL. This would make people wary of using the LLM for writing code, and thus would make the creators of it wary of training it on the code.
Now just because you don't understand a language model and call its fitting procedure 'learning' doesn't mean that it is doing anything even remotely similar. And even if it does then it has no legal responsibilities so if you want to distribute it then you as the distributor need to assume responsibility for all the copyrights such an act could possibly infringe.
There are measures you can take to try to prevent the information from any one code base from entering the model verbatim, by severely restricting the model size or carefully obfuscating the data, but to my knowledge nobody has used any method that gives any proven measure of security.
AI wont include it, user wont know it should be GPL, developer wont know it was stolen.
This is a whole can of problematic worms
But to be honest, this is a non-issue. Copilot (and co) rarely outputs copyrighted code verbatim and when it does, it's usually too small or too trivial to fall under copyright protection. I made a prediction last year that no one will get sued from using Copilot generated code and I believe it has held so far[0].
I also know how they learn. And I know how the biological brain learns. The differences are just technical. The underlying concept is really the same: Both learn by adjusting the neuronal strengths between neurons based on what they see.
Legal responsibilities is sth different. This is up to politics to decide.
My argument is purely on the fact that what humans do and what machines do is extremely similar, and the differences are just technical and don't really matter here. This is often misunderstood by people who think that machines don't really learn like humans.