As I understand it, there are two different issues, copyright in the derivative work, and legality of the act of creating the derivative work.

Copyright of a derivative work that itself meets the standard for copyrightability rests with the creator of the derivative, because, under law, any copyrightable work has a copyright that belongs to the creator at the moment it is created.

A derivative created without proper permission is also a violation of the copyright of the original, but that's a separate issue.

Just saw your other comment and edited, apparently I didn't check thoroughly enough for the US (Here in Germany the permission isn't needed in most cases. Without permission you can't do much with your derivative work afterwards of course)

how can people do these kinds of work on a speculative basis then? surely their work is protected by copyright even though if they were to reproduce it, it would be infringing. I am sure it's in their copyright when they try to sell it to the company they've derived it from.

otherwise how could anyone do spec work!

EDIT: I mean unsolicited spec work - the company they're pitching doesn't even know they're pitching them until they send their pitch. Surely their pitch is still protected by copyright, even though as it's unauthorized the person producing it wouldn't have the right to publish their pitch without approval (if it's a derivative of the company's assets)?