It only became a hard problem to solve because: 1. The creation of the unlicensed audio book was tolerated 2. There is a presumption that the unlicensed audio

It only became a hard problem to solve because: 1. The creation of the unlicensed audio book was tolerated 2. There is a presumption that the unlicensed audio book should be protected from derivative works that profit from it It's hard because the intolerance of derivatives works in (2) is inconsistent with the tolerance of the derivative work in (1); and so raises the question of when do unlicensed derivatives convert from tolerable to intolerable. This is outside of all but the most liberal interpretations of fair use, IMHO.