I agree, some of the Linux people have a very broad notion of what counts as a derived work, but I haven't seen much in the way of actual case law to support th

I agree, some of the Linux people have a very broad notion of what counts as a derived work, but I haven't seen much in the way of actual case law to support the conclusion that "white box reverse engineering is generally illegal". Software generally receives wide protection for its 'non-literal elements', but it's not the case that every possible iota of its function falls under its protectable expression. Indeed, plenty of U.S. courts have subscribed to the "abstraction-filtration-comparison test" which explicitly describes how some characteristics of a program can be outside the scope of its protection. But in practice, many of the big software copyright cases have been about literal byte-for-byte copying of some part of a program or data structure, so that the issue comes down to either contract law or fair-use law (which Sega v. Accolade and Google v. Oracle both fall under). The scope of fine-grained protection for 'non-literal elements' seems relatively unexplored, and the maximalist interpretation has seemingly proliferated from people wanting to avoid any risk of legal trouble.