Up to a point, how much is it other people wanting to spread the product? As author and TM-holder, he has a number of options: agree to things other people do, only agree to what he can personally control or agree to nothing (apart from the books). The problem with the former is it can get out of hand and quality can be questionable, whilst with the latter is you'll get all sorts of unauthorised products, as there certainly have been for other authors (e.g. JK Rowling). It's possibly therefore in the author's own interest to restrict products up to a point (Discworld-brand "Sonkies", anyone?) whilst on the other hand assisting with others that are appropriate, to add detail and keep standards high: the Mappes, companions, diaries, etc. Then again, how much is fan-fed: e.g. who didn't, in the early days, try to draw their own map?