The burden is on the user to show that it is fair use, no? Not everyone else's responsibility to prove that it's _not_ fair use.
Accordingly, anyone on the internet who wants to make comments about how they should be able to prevent others from training models on their data needs to demonstrate competence with respect to copyright by explaining why it's not fair use, as currently it is undecided in law and not something we can just take for granted.
Otherwise, such commenters should probably just let the courts work this one out or campaign for a different set of protection laws, as copyright may not be sufficient for the kind of control they are asking over random developers or organizations who want to train a statistical model on public data.
That said, I think we do agree. The plaintiff should be prepared to refute a fair-use argument raised by the defendant. I'm just noting that the refutation doesn't need to be part of the initial filing, it gets presented at trial, after discovery, and only if the defendant presents a fair-use defense. So they don't have to prove it's not fair use to win in every case. I'm probably also being excessively pedantic!
No, fair use is an affirmative defense for conduct that would otherwise be infringing. The onus is on the defendant to show that their use was fair.
Morally, perhaps, but not under US law: https://en.wikipedia.org/wiki/Affirmative_defense#Fair_use