It is the GM's job to decide the fiction. Players can prompt this, but the GM is the final arbiter.
For example, if the GM says, "The ruler of Neverwinter is Lord Baadguey the Viileen," then the players might say, "Hang on, the SCAG says it is Dagult Neverember." but the GM has the right (the mandate? the prerogative?) to say, "I know, but in this game its Baadguey."
When the players are disagreeing with that, they are, to put it bluntly, wrong.
Player: I talk to Lord Neverember.
GM: No, it's Lord Baadguey.
Player: No, it's Lord Neverember. Also, he's a Harper so I show my Harper pin so he has to grant my request.
GM: No, he doesn't.
Player: This is unfair! How come you never let me use my character's abilities!?
Now, players can certainly misread and misunderstand the game world. In a game I am GMing right now, the players have completely ignored any evidence that their opponents are simply brainwashed peasants magically dominated by four evil bad guys™. This is not going to absolve them of the consequences of killing tens of basically-innocent farmers.
In my games, if the players say "No, he's right, maybe a bit rougher then we like, but lets go talk to and join him" then they are now the villains and the game ends, because I'm not interesting in GMing villain characters.
I also believe it is the DM's job to decide when and if the fiction overrides the rules.
For example, a player declares, "My character does X" and the GM responds, "X is impossible, you can't do it." The player complains, 'But don't I get to make a roll?" and the answer is, "No, you don't."
The game rules say that the players gets to roll a skill check but the fiction (the game world) overrides that because their desired action has zero chance of success. Or 100% chance of success, in which cas the GM says, 'You succeed."