Not much to explain, that's just how Fair Use doctrine is currently interpreted. It's not really morally right in this context ("greater good" rationalists will argue with me on that) but so far the courts haven't spanked anyone too hard for using that loophole.
They take publicly accessible code and learn to predict good token strings from it to solve related or unrelated problems. I am not convinced this is not morally right.
What they are not doing, as far as I can tell, at least not intentionally, is just copying code and removing the license and attribution. Even if you don't see the AI output as legitimately "creative", it's certainly transforming and remixing existing solutions in ways that are transformative enough not to be just copies of anything except the most bog standard boilerplate, which is usually not a copyright issue.
People seem upset because there is money to be made, if there was no money involved I don't think anyone would see any issue here.
It's not about the greater good, and I don't mean to be an apologist, but I just legitimately don't see a problem with doing it, or how it's not fair use. It really does seem like fair use to me.
But it's not forking or linking to any of that software. It's "using it" only in the sense of reading the code as examples of how to do things after mixing it with a million other sources. Sorry but I just don't see the problem with that. Very different, categorically, in my mind from what is intended by software licenses when they talk about derivative works.
