> The library has an interest in Big Scary Warning because the US has an adversarial legal system which makes it easy for people to cost other people $$$$ on legal fees about stupid shit.
I think this is unlikely here. I would expect such a suit to get tossed out in the early, early phases because the plaintiff doesn't have standing.
Per Manuel LUJAN, Jr., Secretary of the Interior, Petitioner v. DEFENDERS OF WILDLIFE, et al. the 3 part test for standing is:
1. The plaintiff must have suffered an "injury in fact," meaning that the injury is of a legally protected interest which is (a) concrete and particularized and (b) actual or imminent
2. There must be a causal connection between the injury and the conduct brought before the court
3. It must be likely, rather than speculative, that a favorable decision by the court will redress the injury
They almost certainly fail the first test. Making bootleg copies of stuff to sell is not a "legally protected interest".
They also likely fail the second test because having copiers available, even without signage, isn't causative of copyright infringement.
The third is largely speculative, but I would bet a case like that fails that too. I don't even know what law someone would sue under for "they failed to tell me what the law is". Citizens are expected to know what the law is (whether that's reasonable or not is another question), as evidenced by the many laws that don't require mens rea.
In all likelihood the way this goes is that person files a lawsuit against the library, the library files a motion for summary judgement, and the judge rules against the plaintiff before discovery even happens. The facts of the case are irrelevant, because even if we presume that everything the plaintiff says is true, the library wouldn't be liable.