Seems to me it's about protecting the copy right.  By protecting copy rights, I mean the characters/places/etc contained in the books which would likely be mirrored in fanfics.  Xerox and Kleenex are no longer copy right held terms because they became such a part of the commonly used language.  Google will be next if it's keeps being used as a verb.  Most copy right holders have to pursue any infraction to their copy right or face loosing the "value" of the copy right if they don't do it enough times.  By ignoring the fact that fanfics exist, an author protects his intellectual property rights and doesn't have to worry about the future litigation of the fanfic writers who could theoretically limit the amount of plot choices available to him/her.