The more that the GM is just making stuff up, without prep as some sort of constraint, the more that it makes no sense to describe the GMing as "fair" or "neutral", and the closer the game is getting to what Lewis Pulsipher was criticising as GM-as-storyteller back in the early 80s.
This isn't the case. The issue isn't volume in a sandbox (though there are definitely GMs who want to make up less during play). The issue is the rationale and logic behind what they are making up. Storyteller play is about the GM making up a story that the players don't have any meaningful impact on. If the GM is enabling the players to make meaningful choices, including not engaging the story, and the GM is legitimately trying to remain true to the setting, to the motivations of his NPCs and groups, and creating things that he feels fits that truth, that is outside the storyteller GM approach