The main point of my argument, and the main concern I feel with GPL-style licenses, is the notion of "do I have to distribute my work under the terms of the GPL even if I don't distribute any GPL code directly; in other words, when is my program a derived work of a GPL work?". If instead I am distributing GPL code directly, the question is more clear - as you say, copyright law makes few exceptions there.

But again, if my program doesn't directly include any code at all from a GPL work, but it does depend on such code to exist on the end-user system, knowing in what circumstances my code is a derived work and thus can only be distributed under the terms of the GPL is much harder.

This is exactly the case of the kernel modules: the people distributing the kernel modules are often NOT distributing Linux itself. But, can their modules be considered derived works of the Linux kernel?

Note that this kind of question can also arise in traditional copyright. I can make a play that very much resembles a novel, and depending on many ultimately subjective factors my play may or may not be considered a derived work of that novel. If I am directly including substantial parts of the novel, then the answer is pretty clear, but a work that doesn't have a single word of the original can still be a derived work (most obviously, a translation is a derived work, but it likely literally doesn't include a single word in common with the original work).