I'm not even talking about adhering to the standard D&D rules, guys. I am talking about scenarios where there could be anything of the DM's choosing behind a door. Monsters, traps, treasure, a fair maiden. They can choose how to have their NPCs respond, short of diplomancer abuse. They can either let you buy that diamond to resurrect your friend, or perhaps some thief happened to steal the last one. What monsters you encounter on a journey, if they bother using a random encounter table, if they even bother throwing an encounter at you at all is all up to the DM.

Now, what if the DM was doing their best to be fair and make the most logical outcomes from some players' actions, but they still thought something was unfair, like events were being manipulated? This could arise from a number of factors they simply don't know.. because they simply aren't the all-knowing DM, or it could just be paranoia.

Now consider a DM that is pretty deadset on manipulating the party into taking a certain set of actions... and that party is totally oblivious to that manipulation, thinking it's the best sandbox ever?

As I said, it's the illusion of meaningful choice that makes a sandbox. Not that games don't exist, or players don't exist, or the concept of choices don't exist.