I hope nobody minds that I'm discussing a three-month old posting, but I've wondered about these questions myself. Like Vexx, IANAL, yada yada, though I have taken an interest in copyright issues because of some work I did in the 80's.
I would think that doujinshi fall more into trademark infringement rather than copyright infringement. If I draw a picture of Kagamin, I haven't actually copied anything, so I don't think my drawing can be seen as infringement of copyright. However most characters are also trademarked. I visited the most obvious site I could think of that might shed some light on Japanese characters, namely
Sanrio. Along with the discussion of copyright infringement, there's this paragraph:
"The trademarks, logos, and service marks displayed on the Site (collectively the “Trademarks”) are the registered and unregistered trademarks of Sanrio, our licensors and suppliers, and others. Nothing contained on the Site should be construed as granting, by implication, estoppel or otherwise, any license or right to use any Trademark without the express written permission of Sanrio, our licensors or suppliers, or the third party owner of any such Trademark and use of the Trademarks is expressly prohibited."
So I can't copy a picture of Hello Kitty off the site; that would be copyright infringement. Nor can I publish a picture of Hello Kitty; that would be a trademark infringement.
Interestingly, in the US I believe your required legally to pursue trademark infringers and that failing to do so can render the trademark indefensible. I don't know whether Japan has the same law in this area. (Trademark law varies more from country to country than copyright law.) If so, then it would be dangerous not to pursue doujinshi publishers.
I'd agree with the comments above, though, that the free advertising in doujinshi may benefit the original creators more than the costs, and bad PR, of pursuing actions against them.