I think 'scope' may be a bit of a red herring. Every game has some sort of boundaries of what is and is not in scope in some way. However, I would say that scope is an agreed on thing. I mean, if the game we all decided to play was 'Zombie Apocalypse Survival Horror' set in 1950's rural Midwest USA, then I would not expect to incorporate out of genre elements, nor that my character's main goal is to become a wizard, etc. Within that 'box' however, there can exist a Story Now game, or a game that is GM-Directed based on pre-established setting elements (backstory first as
@pemerton has labeled it). So, yes, we agree, and I think it is non-controversial and core to RPGs, that its not 'force' when a genre/setting constraint is invoked on an agreed-upon basis. I even think that these adjudications can be partially made during play, so that the constraints are refined, although I think making that the sole realm of the GM could prove to be a problematic design element in a game.
I mean, you can define things down as much as you want, but at a certain point it gets to be more than some people want. I'm not wanting to play in an AP where I have no input to do things that are otherwise genre appropriate and seem 'in the box', simply because the game is only going to focus on specific locations and goals. At least, I am surely going to want that to be negotiated up front, explicitly. In that case, actions that derail the AP are 'out of the box' and there's no need for a GM to be coy about that. Since presumably the story is not revealed beforehand, they will then be in a position where they may make such statements and it isn't really negotiable, because it relates to unrevealed backstory. OK, that's fine too, its an agreed type of play, but note how there is a lot there! I think it makes sense to have a term for that kind of play...