Because you leaven that impulse with other ways to make decisions. I use a fair number of tables for this purpose. Everything on the table is still logically consistent, but it's not all equally plausible.
And that’s what’s being suggested as a way to alleviate the issue, when folks see it as such.
Tables and dice rolls and the like help remove the matrix of GM fiat that would otherwise determine nearly the entirety of the situation, as well as the resolution.
But again, that is not so simple. Some sort of GM decisions might indeed erode player agency, whits others could empower it. Like I have been talking about information that is at least potentially knowable to, and actionable by the players. "There exist this specific assassin sect with an ability gained via a defined method that allows them to walk through magical defences without trigging them" is that. "The dice say no" really isn't.
Potentially yes. But knowable how? Is it hinted at by the GM at some point? Is a skill check of some kind used? Is the GM obligated to share these details about this sect? What prompts the players to wonder about such things? How secretive is the sect? Would their characters already know about the sect or not?
How are all these things determined?
For some reason you seem to be perfectly fine with the dice saying no, yet have an issue with the GM saying no to the exact same thing. In either case, the player action declaration was nullified, so their agency was eroded just the same, right?
No. That’s not how agency works. Agency is not the guarantee of success, it is existence of the possibility for success. I’m free as a player of basketball to take a shot from beyond the 3-point line. I may miss the shot… it’s farther than a 2-pointer, and that’s the risk I’m taking. That’s me as a player exercising agency.
Typically in RPGs, dice or some other randomizer are used to determine the outcome of skill checks such as that. Understanding these things is the key to player agency.
Jack's agency is up to Jack, not a GM.
It is just that if the GM has coherent reasons for their no, then those can become leverageable by the players, thus allowing an avenue of agency, in a way that cannot happen with mere randomness.
I think you’re off here. You say these world building details are available to find out before hand, right? I'm guessing that would most likely happen through the standard resolution system of play, right? So the DM would consider if it was uncertain.
So, he can decide it's certain that the PCs know of this sect. This will then inform how they handle their long rest, especially if they have reason to believe one or more of these assassins is on their tail.
He can decide they don't know at all... it's a super secret sect and there's no common knowledge of their existence. The GM denies the knowledge. This is a world building detail that contributes to the camp situation that the PCs cannot act on prior.
Or he can call for a roll of some sort. Hopefully involving a sensible DC and choosing to share the necessary information based on the result.
Now... whatever prompts the above in D&D... one of the players is worried about being attacked at camp, or worried about the amount of enemies the PCs are making, and the likelihood that someone may hire someone to deal with them... the player says something like this to the DM, and the DM calls for a roll.
Why can't this also happen in a game where this information is not all determined prior to play? I haven't played Torchbearer 2, but I have read it, and I've played Mouse Guard, which uses the same core system... and I'm reasonably sure that it can.