They hold the copyright on the text and any images contracted by the authors or authorized agents. How can they control images inspired by that text (non-visual reference) when such are not "Work for Hire" pieces? As long as you are not close to any of the authorized published works in your design you should be fine. IANAL (std disclaimer) but it sounds as if they are just trying to intimidate.

Pass no temptation lightly by, for one never knows when it may pass again!