The only time to directly threaten legal action is when you don't think legal action will achieve your goal, and you've got nothing else. Otherwise, there is no benefit to threatening legal action. Sure, you can imply it, but you gain nothing by showing those cards.
But the goals that these people pursue are difficult to understand or anticipate. You assume that because these people are employed by some business entity that their goals are "make more money" and such. That's rarely the case, even if we allow for the unenlightened version of "make more money". Instead, assume that many, possibly even most, are more interested in being punitive out of some warped sense of justice.…
The likelihood of a non-profit outfit building open source software being driven by a warped sense of justice and the goal of making people miserable rather than, say, trying to make sure they can continue their stated mission and ensure publishers keep agreeing to digital book lending is pretty bold. But sure, why not.