In my opinion, everyone at the table is relying on everyone else to not be a douche and to contribute to the fun rather than detract from it. I believe this is an unwritten social contract people make when agreeing to get together for a game.
He was of the opinion that, as a DM, if he sets something in front of the players, then it is what it is, no matter what. If he's decided it's a dead-end, then it's a dead-end and he won't alter that for anyone or anything.
I don't see how these statements are mutually exclusive.
Everyone at the table agreeing to be polite to one another, and not to ruin the fun of the guy next to you, has nothing to do with whether that hallway is a dead end or not. One is Gamer Etiquette, the other is DM Philosophy.
By that I mean that, sure, everyone has their own way of doing things, but at the end of the day, there are some pretty fundamentally key aspects to it that should be shared across the board.
I think that there are fundamental aspects that DMs share in terms of opinions. Or, simply, fundamental aspects that make a DM good or bad.
As to the specific situation, and extrapolating on it, there may be a very legitimate reason why
a DM might choose to simply make it a dead end: lack of preparation. You site a "Water hazard, two traps, and an ooze"; traps, a water hazard, and an ooze of the appropriate level may require a bit more forethought and looking up rules than just whipping out a book and having it right there in front of you. There are only a few traps and hazards in the book, so you can't have one (even reskinnable) for every situation.
Some DMs also are just
not good at improvisation, especially Action Improvisation. If the PCs on the moment's spur decide to invade the Lord's Manor, the DM now has to scramble for a battlemat and stats, or think of some sort of appropriate situation; it's easier to just say "No, you don't." Or to smack the PCs down. It's easier to just say "It's a dead end", because the DM has nothing in their pocket. After all, how does the DM change the map on the fly?
Granting the players a reward requirse you checking to see "What's balanced, what's appropriate for these guys, etc".
There's also a certain level of
Fear of Reason. Your suggestion is that a DM should toss the players a bone, give them something story-wise for what they're looking for. But that something - the first thing the DM thinks of - might make
no sense whatsoever, or lead to questions the DM has no answer for. If the PCs decide while walking through the State Capital Building, "I'm going to search for hidden stuff!" and the DM says, "Okay, you find a secret door", now the player says, "Okay, where does the secret door go, why is it there, who built it, who knows about it, what can I do with it" - the DM isn't
ready for that.
Then there's the issue of red herrings. Even if you toss the PCs a bone, they might go way, way off course, for all the things you've prepared. See preparations earlier.