So, I write fan fiction, and I was curious to know just how bad I was being. I asked a few people online if fanfiction was copyright infringement, even if one had no intention of making money from the writing. I got a great resounded "Yes". They said that fanfiction presented a "competing product" to the original, thereby infringing on protected material.
How can it complete with the original if you aren't making money?
In an indirect manner--if the fanfic is good enough, what it can do is shrink the available market for tie-in literature. I myself rarely buy official Treklit anymore, because I think that there are fanfic authors out there who are offering a better product and
at no charge.
As to the shows, movies, DVDs, and other types of merchandise, fanfic is fuel to my fandom. So really the question depends on if you're a Treklit publisher, or not.