I disagree with your framework here and think it's problematic in a few ways.
I think your positioning of the game designer as a God figure is problematic. The game designer does indeed make god-like decisions about the world they are creating, but they lack the cohesiveness of the Divinity (what is actually represented as a "Game Designers" choice is actually reflective of a multitude of other people, and might not actually reflect what the Game Designer wants.) Similarly, the game designer must make strategic decisions about the aspects of the game to include, and which ones not to. While you frame the choice to not let the player stand on the horse instead of it as a malicious decision designed to force you into a specific mode of thought, it is reality more likely to be a matter of resources--A God figure would have unlimited time and resources, taking anything strategic out of the equation--which makes all decisions necessarily moral decisions because there are no other considerations. You cannot proscribe morality onto a decision that could just as easily be a strategic/resource driven decision as it could be a morally guided decision. (I'm using morality here in such a way as to imply a deliberate effort on the part of designer to ensure that the game is played in only one way, the way the designer intends.)
Finally, player agency really has nothing to do with being in line or out of line with the designers intentions (or, in other words, their morality). In fact, within the context of an open world, player agency effects designers exactly as much as it does within more structured, rigid worlds -- ultimately, the player can only act according to how the designer dictates and never in any other way.
Within a game, the concept of Free Will is an illusion -- the game can never be anything other than a pre-destined outcome. In essence, you've completed a game as soon as you've purchased because there is no other possible outcome except to win. We often like to think that we can "Lose" a game, but we can't. The best you can do is "Not Win yet," and even implies the eventual outcome of winning the game. Free will within a game is counter-intuitive because by playing the game, you are tacitly agreeing to the rules of engagement - that is, giving up your free will to act in any way that strikes your fancy, and instead decide to act in the way the game proscribes for you.
I think the sole exception to this would be something like SecondLife, where the game is literally defined by the player. The rules of engagement are defined by the player. But even within SecondLife, you can join other games where your free will is essentially stripped. SecondLife itself only provides a greater illusion of free will than other similar games - but never the truth that is free will. Only a game that contains no rules and no guiding principles can let you truly express your will, but then the game, which is essentially a set of behaviors guided by rules, actually ceases to be a game.