The reasons are clear enough: in an attention economy, the key is to capture customers and keep them focused. The dojinshi market does exactly that. Fans obsess; obsessions work to the benefit of the original artist. Thus, were the law to ban dojinshi, lawyers may sleep better, but the market for comics generally would be hurt. Manga publishers in Japan recognize this. They understand how “theft” can benefit the “victim,” even if lawyers are trained to make the thought inconceivable.
Via dive into mark [1], “What lawyers can learn from comic books [2]”
I've linked to [3] a few other [4] articles where making intellectual property (books, music, comics) easily available helps sales in the long run, even if it may facilitate an apparent “pirate market” in the short run. And this article by Lawrence Lessig [5] expresses that point all the more so (and while he is correct in his reference to the potential legal action [6] by Sony [7] against someone who hacked the Aibo [8], I can see Sony's side of the picture—they were trying to limit their liability if someone saw the information, hacked their Aibo and broke it, then tried to return it to Sony possibly despite langauge in their warantee that modifications to the Aibo will void it; Sonly has since changed their mind [9]).
[1] http://diveintomark.org/archives/2003/01/14.html#in_brief_14_jan_2003
[2] http://www.redherring.com/insider/2003/01/copycats011003.html
[5] http://cyberlaw.stanford.edu/lessig/
[6] http://www.infosatellite.com/news/2001/11/a051101aibo_hacker.html
[9] http://www.infosatellite.com/news/2002/05/a080502aibo.html