People who train AI models claim that anything they can find on the internet is fair use.

It's the whole point of the discussion… is it really?

And if it's not fair use to train on windows source code because of copyright… doesn't that same copyright law cover everything else as well?

I think there's a reasonable distinction to make between "you can train AI models on any code that you are legally allowed to have and read" and "you can train AI models on any code that you are able to feed into it, regardless of whether you have permission to possess/read it".

You're not legally allowed to have and read it if you knowingly violate its license terms.