adh1003: "The reason why a royalty is payable for commercial RISC OS [Open] users is partly because those users can keep their code or modifications to core RISC OS private. They pay for what amounts to exclusivity. It's about control in a way, as you said - but not Castle's control, as you indicated; I believe it's actually there to grant more control to the user of the code."
So what would be the problem with a choice of the GPL or some closed source licence? That someone could still sell GPL-licensed code and Castle wouldn't see any of that money?