That's how copyright works. To create a derivative work, you need a license. It's no more derivative than the works the authors produce, publicize and sell on

That's how copyright works. To create a derivative work, you need a license. It's no more derivative than the works the authors produce, publicize and sell on their open source software platforms. Are they also going to be charged for every book released that has in some way contributed to their cultural hinterland? We're talking about building a cultural context en-masse - not taking individual books to enjoy for free and plagiarise. That single vs mass use of the books makes a significant ethical difference.