QUOTE(lightshader @ Jun 7 2012, 00:07)

To be honest, the doujin themselves are gray matter to begin with, since artists are using JAPANESE copyrighted material without permission to begin with. But this is generally overlooked because it has some benefits of bringing in new pool of artists and free advertising. Even so, artists really don't have much ability in terms of stopping this kind of scanning and distribution of their works - unless it's an original work, they really can't claim the content inside their works to be their own (since they're based off copyrighted works in the first place).
Whether doujinshi use copyrighted material or not is a gray zone in itself; it's not at all clear that a setting or likeness of a character is copyrightable. Ideas are not copyrightable, but expressions of ideas are. However, this obviously doesn't matter if you're "infringing" on someone with a shitload of money (Disney, for example). Furthermore, in most countries, rights holders can't claim infringement against someone using their work to criticize or satirize it. (Which doesn't stop them from trying anyway though.)
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What any meager semblance of protection over their works in Japan they have, outside Japan they have no control because any form of copyrighted material are copyrighted in Japan - unless the copyright holder has also bought copyright protection in other countries such as United States.
Your understanding of copyright is completely wrong. Copyright doesn't know borders and isn't bought; it's granted, globally, as soon as you create anything that lives up to certain (very low) standards of what may be considered an original work. See the Berne convention for more information.
Either way, since there's practically no money in doujinshi, nobody's going to have the will or lawyers to do anything about it. I'd worry (marginally) more about published H manga.