Like I said, the cause is probably the doujin community itself, which bases around using other people's work and rereleasing it in another fashion. To use stuff like that is a sign of respect and they probably didn't know better that we have different rules regarding using material. To have credited him shows that enough already. Either way it doesn't really matter why they didn't contact him, it just needs to be resolved now.
Is the non-commercial part of the Japanese version of the license (legalese version) similar to the English version?
Link here. That is, of course, assuming that the developer didn't try going off of the English version.
Putting the fact that English isn't the developer's first language aside, I wonder if the doujin community's definition of "non-commercial" (and perhaps Japanese law, I dunno) was part of the reasoning that the developer used to determine that using KgZ's music and giving him attribution was sufficient. There's obviously a plethora of works based on anime and manga properties that, due to the scale and nature of most of the doujin material in question, isn't sufficient to draw the wrath of the studios/publishers that own those properties.
Something less ambiguous is that the license also mentions (in the legalese bit) that:
You may not exercise any of the rights granted to You in Section 3 above in any manner that is primarily intended for or directed toward commercial advantage or private monetary compensation.
While I believe most doujin authors are probably trying to make a little money that could also be left open to differing interpretations if an author is simply recouping the cost of actually making the work available (i.e. costs to cover printing a manga, or cases/CD pressing/manuals, etc.) instead of making a profit. Some licenses like the GPL spell out that you're able to charge a fee to recoup those kinds of costs.
