So, if I'm understanding this correctly, is the argument is that providing the user with a choice of library to link to, effectively "induces" the user to violate the GPL?
I'm using the concept of inducement as a theory of contributory copyright liability, even though it wasn't a concept of law at the time that RMS and Bruno had this discussion. It feels like an inducement argument, though - the idea that telling your users to break the license is just the same thing as breaking the license.
However, I don't think Bruno's argument hinges entirely on telling the user to break the GPL on his behalf. There is an API boundary, and in the time following this argument, people have written actual readline replacement wrappers that use BSD-licensed line-editing libraries (e.g. editline). The way I interpreted Bruno's argument was with the implication that he would actually reimplement readline instead of just, say, writing a functionality-free shim to induce the user to replace it with a functional but GPL'd library.
Not inducement but rather intent. The question would be what the genuine intent of the author was behind the decision to so conveniently choose to place an API boundary right in that particular spot. The precise layout of the API also seems like it would be relevant - if no non-GPL libraries conforming to that particular API existed at the time of the decision, then what would the non-infringing justification be for choosing to implement that specific API in the first place?
(Aside: In Google's case, IIUC the non-infringing justification is code portability. Also, there's the bit where they (re)implemented the entire system - I don't believe they were linking against or otherwise using anything owned by Sun. Other than the API layout itself ... allegedly.)
Presumably questions about the decision making process itself, the technical merit of the final decision, and the state of any alternative implementations would all need to be considered.
I'm using the concept of inducement as a theory of contributory copyright liability, even though it wasn't a concept of law at the time that RMS and Bruno had this discussion. It feels like an inducement argument, though - the idea that telling your users to break the license is just the same thing as breaking the license.
However, I don't think Bruno's argument hinges entirely on telling the user to break the GPL on his behalf. There is an API boundary, and in the time following this argument, people have written actual readline replacement wrappers that use BSD-licensed line-editing libraries (e.g. editline). The way I interpreted Bruno's argument was with the implication that he would actually reimplement readline instead of just, say, writing a functionality-free shim to induce the user to replace it with a functional but GPL'd library.