If no one’s making money off your fan fiction then you shouldn’t give it so that someone else does. HBO are offering resolutely poor terms and conditions for your contributions. Martin stopped the fan fiction being published on Amazon so I’m not sure why he’s allowing this to happen.
Make your own worlds and control them. Make better art that’s yours and ask people to contribute towards you being in a position to make more of it. As artists, we’re traditionally bad at managing the business side of things but that’s where we get screwed over.
If you’ve looked at the hacked emails of the Sony producers then you can get an idea of how they feel about the actual artists without whom they wouldn’t exist.
We can help each other. Art isn’t a zero sum game and we’re not in competition. We’re not enemies but the illusions of scarcity allow for such disparities.