Is there actually any evidence that derivative works are damaging to the original? If anything, they serve to advertise the original, making it more well known, or even fueling the subcultures that arise around a piece of art.
If you're not familiar with it, have a look at the Touhou project (set of Japanese arcade-style games, where the author explicitly gave permission for people to remix his art, whilst retaining fully copyright over the games, and forbidding commercial distribution of them). Touhou is huge - and it's not just because of the games - the derived art works have significantly impacted take up of the games, and created a somewhat unavoidable franchise, inspired countless of creators to produce and share, and will probably have a long lasting presence within otaku culture.
It's not to hard to see a situation where BigCo could undercut an artist's own sales due to their economies of scale, particularly if the moneymaking part of the art was in merchandising (such as for webcomics).
He is now allowing derivative games because they are protected by copyright, however, his artwork is free to use with few exceptions, so there's nothing preventing you from creating new games using the same characters, which are not derivatives, as many have done: http://touhou.wikia.com/wiki/Other_Games
In books, this would be equivalent to reusing characters, their personalities or traits, but copying sections of the original works verbatim is obviously still plagiarism.
If you're not familiar with it, have a look at the Touhou project (set of Japanese arcade-style games, where the author explicitly gave permission for people to remix his art, whilst retaining fully copyright over the games, and forbidding commercial distribution of them). Touhou is huge - and it's not just because of the games - the derived art works have significantly impacted take up of the games, and created a somewhat unavoidable franchise, inspired countless of creators to produce and share, and will probably have a long lasting presence within otaku culture.