I agree fan fiction doesn’t take away from the original works. I personally don’t touch the stuff (well, non-H, anyway) because I prefer the art the original artist created, not so much the cover band version.

From what I understand, most non-porn doujinshi are not based on a pre-existing intellectual property and are simply mangaka trying to break into the manga market through nontraditional means with their own story and characters — kinda like indie comics here in the States. I’m sure that there are those non-porn doujinshi that do steal both characters and situations from another artist’s series, but from what I gather, these are the exception rather than the rule.

And as far as I’m concerned, if an otaku already has every single extant volume of a finished series, he can’t spend more money on that series — as made by the original author — because there isn’t any more. Thus he turns to doujinshi to fill that void, much like how since Star Wars is over people turn to the novels, comics, and fan films. Since the novels and comics aren’t written by George Lucas, they’re licensed, and can thus be sold, but doujinshi are basically unlicensed fan films that people pay for. I’m not saying it’s the artist/publisher’s fault for not trying to license doujinshi material, just that the market exists.

I’d be more fine with derivate non-porn doujinshi if the artists gave them away for free (or at the very least, sold them at the cost of materials and printing) or sought out the original artist/publisher (who probably isn’t interested in continuing a series that has already been laid to rest) to talk about some kind of licensing or royalty deal (i.e. you can sell our IP so long as you give us 5% of your sales).

P.S.: Most doujinshi aren’t porn. On Comic Market, it’s estimated that only ~30% of publications have sexually explicit content.

1) Doujinshi ultimately aren’t worth the trouble of fussing over.

In the rest of the world, most fiction writers actively denounce and prohibit “derivative fiction” or “fan fiction” for legal reasons, due to several cases where fanfic writers actually sued the original author, claiming the author stole their idea (the case with Marion Zimmer Bradley comes to mind).

However, a doujinshi based on a previous work isn’t trying to claim ownership of the characters or story and doesn’t have any plot worth stealing because it’s meant purely for titillation or amusement.

2) Doujinshi act like free advertising.

Statistically, the more popular a series is, the more doujinshi will be made for it. The number of doujinshi sold alongside the original work in bookstores will, in theory, be directly proportional to how popular the series is, which may ultimately influence someone to purchase the original work by popularity alone. Any business worth their salt loves free advertising, be it word-of-mouth buzz or anything else.

3) Doujinshi fill a void in the market that publishers can’t (or won’t).

Let’s be honest: Sex sells, and most doujinshi based on an existing work will be rife with it. I think if Japanese publishers could get away with, say, publishing both a manga series aimed at preteens and a series featuring the same characters but in decidedly adult situations, they would, since the market for it clearly exists. But doing so would be like an elementary school running a brothel in the back room. Doujinshi ultimately fill this void in the market without the original publisher getting their hands dirty, so to speak, but although the publisher doesn’t see a dime from their sale, the doujinshi themselves being a testament to a series’s popularity is, again, free advertising.

4) Doujinshi are parodies, which are not illegal.

When it comes right down to it, most doujinshi are considered parodies of the original work. This is the same idea as a pornographic movie that uses the same plot (if any) and characters (or character concepts) as a popular blockbuster movie, even going so far as using a derivative, satiric title that lets people know exactly what the original work they are imitating is. The only real difference between movie parody and doujinshi is doujinshi are imitating an established art style, whereas a movie parody cannot make clones of the original actors to use in their film and thus have to use different ones.