They aren't even vaguely equivalent. One is commercial, the other is non-commercial. One competes in the market with them, the other doesn't. If the kid decides to give a concert, charge admission, and not pay royalties, that would not be fair use.
Fair use is based on four factors.
1. Is the copying for commercial purposes? Commercial use is less likely to be fair use than non-commercial use. The AI companies fail on this one.
2. How much are they copying? Copying just a small piece of a work is usually fair use, while copying a larger amount is less likely to be. The AI companies are copying tens of thousands of songs in their entirety. They fail on this one.
3. Does it compete in the marketplace with the original work? AI companies have signed contracts with lots of other companies to pay large amounts for the rights to train on content. There's a large market for training rights. Pirating the works instead of paying for them competes with the plaintifs. They fail on this one.
4. Is the use transformative or just plain copying? In this case, it's just plain copying. I'm not talking about the models they produce (there's an argument about whether they infringe the copyright) or the outputs of the models (which also arguably infringe the copyright). I'm just talking about the raw data used to train on. They copy it directly because they don't want to pay for it. They fail on this one.
Since they fail on every one of the four criteria, there just is no credible argument that it's fair use.