Is there data that supports this? I’d be interested to know what % of people who buy a Cliffs Notes have already _bought_ the original.
But, anecdotally, it's what I've seen to be the case.
Yes.
For example, Wikipedia cites many research journals that otherwise are available only by subscription.
Prior to Wikipedia, gated information centers were the norm.
The question was NOT whether it spreads information from the articles to people who wouldn't have paid for it. The question was whether it suppresses sales of the articles to people who otherwise might have paid for it.
That's a more complicated question of fact. Some people now read Wikipedia and won't buy the article. Some people encounter the reference on Wikipedia and decide to buy the article. Which happens more?
I don't have data. But publishers do. And https://scholarlykitchen.sspnet.org/2022/11/01/guest-post-wi... shows what publishers concluded.
Publishers concluded that Wikipedia references are good for sales. And so jumped on the chance to cooperate with https://wikipedialibrary.wmflabs.org/. Which is therefore able to give free access to 90% of subscription only databases to you if you can prove that you're the kind of person who is likely to add citations to Wikipedia.
Legal questions are funny like that. You have to answer the question actually asked. If you merely answer another one that sounds similar to you, your answer is generally wrong.
Second, you ignored the "purely derivative" bit. You have to look at to what extent the use is derivative or transformative. See https://en.wikipedia.org/wiki/Transformative_use for a bit about that. (Note, this is a legal term defined by various precedents. OpenAI can't just argue, "Turning it into an LLM is a transform, so it is transformative!") Since CliffNotes is educational and Wikipedia is nonprofit, it is relatively easy for both to qualify as transformative.
As a result your response underscores the point that was made. There are a lot of shades of grey. You really can't just seize on a couple of phrases and key points, then jump straight to the answer. You have to understand how the courts will decide, and then accept that there is an actual judgment call whose outcome depends on the judge judging.
(I'm not a lawyer, but I have had excessive exposure to them in the past.)