I suppose this is true, but since people who write fan fiction are often trying to "fix" things that bugged them or expound on things very little explored, I would venture a guess that most companies/authors don't mind the fact that it exists as an outlet for fans. Making up stories about Aragorn's childhood may not be something that I would enjoy, but I don't think it's in any way disrespectful to J. R. R. Tolkien or damaging to the tales he told.
However, writing erotic fan fiction with the sole intent to make money off of someone else's hard work is disgusting. It's IP theft and should be dealt with accordingly. That's my opinion, anyway.
As Celan mentioned above, this was also done by someone who wrote Dragon Age fan fiction and then changed all the names to sell it. Here:
http://www.amazon.com/The-Commander-Den-Asaan-Rautu/dp/1450265790/ref=sr_1_1?ie=UTF8&qid=1374877214&sr=8-1&keywords=the+commander+and+den