Even more important legally speaking: a creator of an original work, whether that be a big company or an author, who does not actively seek to protect his copyrighted and often even trademarked names, warning off any violators of their copyright, stands the chance of losing the trademark and/or copyright. Remember what happened to aspirin. It started out as a brand name for a product against headaches. But the original company forgot to protect the name, and other companies started using it as a general description. Suddenly everybody in the pharmaceutical industry was producing aspirin. The original company that invested so much money in launching aspirin had nothing to show for its efforts.
There is a third reason for the creators of original works to be weary of fandom: the fear of being sued. That is right: more and more fans are taking their own heroes to court because they feel that what they created as derivative work has been stolen from them and used by the original creator in subsequent books, movies, etc. Sometimes it can be as simple as a general plotline that in fact is rather generic, or using something that is logical in the setting or on the basis of the development of the characters; in other words it did not require much sweat from the fan to create, and yet to some it is enough to at least threaten to sue and try and share some of that hard earned wealth of the original creator. We have to to be honest and face the sad truth. Not all fans are innocent victims of their fandom.
The possibility of fans suing over the rights of derivative work has led to some publishing companies advising their authors not to openly participate on the internet, and other authors withdrawing voluntarily from fan newsgroups that they used to be fairly active in. This chilling development makes the internet a lesser place than it used to be, and certainly is not in the best interest of fans.
Under the pressure of their legal departments, some of the big companies use a so called "Big Foot" letter when confronted with fan sites, informing the fan that there is an infringement of copyrights in their opinion and threatening to shut down the site. All of this in a harsh and punitive tone that seeks to subdue the receiver, who usually backs down as most fans do not have the funding or the inclination to face a trial. Remember that the content of the Big Foot letter at this stage of the legal process has not been proven in a court of law, and the concept of Fair Use is usually ignored. The assertions of the lawyers of the big company might not hold up in court, but would you risk it?
At the very least such a letter can be a traumatic experience for a fan, who after all only seeks to enjoy the product more fully. When the "Big Foot" letter is actually followed up with legal action, the fan faces dire consequences; such as steep fines.
It is not in the company's best interest to use "Big Foot" letters and similar overdone legal tactics. It makes their own fans turn away from them, and can give some very bad media coverage, all in all resulting in less revenue.