I'd agree with you, and with @TwoSix. I wasn't suggesting that there aren't limitations in place - different systems have all manner of rules, spoken and unspoken that are designed to limit or guide GM judgement. However, what is the case is that judgement on the part of the GM, agency if you will, or force, exists in every game at every table. It's also true that the extent to which a given GM conforms to those rules varies by GM and doesn't necessarily conform to the intent of the rules set. I'd like to get past the ephemeral fantasy of the entirely impartial GM because I think it makes talking about what matters a lot more difficult. What matters is the nature of the limitations on GM judgment, and I would submit that the nature of the judgment and what limitations apply are more a factor of the actual GM then of the rules set in question, however much the choice of system might betray a certain leaning one way or the other when it comes to ideas of force and agency.
I have a thought, which I have explored some but not extensively, that one can tell a lot about how principled a GM is by the constraints he puts on his own behavior. Not exactly how many, but which ones.
Seeing it, it seems kinda lacking, but I did say I haven't explored it much (even my my standards).