Can reasonable people disagree about that?
I mean maybe? I think someone who disagrees with that is wrong, and frankly, if they told me that at an actual table I would very much feel they were offering fig-leaf excuses for poor DM behavior.
Consider - what if there was a clear houserule where the ability was supposed to work as you envision. And this question goes as much for those more aligned with my views than yours.
I'm not sure what you mean. What kind of house rule enforces "respect the spirit of the rules and the player's intentions"? And if that's not the kind of house rule you mean, I don't see how this applies.
If the DM failed to honor that houserule the biggest issue we would be talking about would be his breaking of the rule.
I consider that a (very badly) lossy compression of the fundamental complaint. The DM denied the use of a player-facing mechanic as intended, by both the spirit of that mechanic and the player's described intent. Yes, at a lossy level, the problem is "you broke a rule." I prefer to be more specific. "Don't break rules" is not a very useful conclusion to draw, especially since
some amount of flexibility is useful even in rules-heavy games. "Don't subvert the spirit of the rules and player intent without extremely good, well-supported justification" is a much better lesson.
What's the element of the scenario that causes that complaint?
The player took an action, with the (explicit) intent to cause certain results. The DM (explicitly) approved that action. The DM then proceeded to actually refute the player's (explicit) intent. The player was
functionally told no. That is MMI, just abstracted away from a direct statement of "no you can't do that" and into "sure you can (but it won't do what you want.)" And then,
because they were told no, consequences followed that were both (explicitly) undesirable and (explicitly) the exact opposite of the player's intent occurred, and the players were afforded no opportunity to do anything about it, despite having plenty of chance to do so. That is the railroading.
I can't speak for him but I am not. What I would add is that if you view Mother May I and railroading as different things then please provide an example where Mother May I takes place but not railroading. That would go a long way for me in understanding the nuance of your position.
Railroading without MMI: Illusionism, quantum ogres, most but not all instances of fudging (whether fudging dice or fudging stars) to ensure a predetermined outcome. None of these involve the players being given permission to do something nor neutering something they were technically allowed to do, but they still ensure that the rails are adhered to.
MMI without railroading: Being a curmudgeonly DM without any planned plot. E.g., running a sandbox but saying no to 90% of player stunts, my aforementioned example of Paladin alignment in 3e ("mother may I keep my powers?" "Nope!"), setting crazy high TNs/paltry rewards for anything that doesn't sound "realistic," etc. None of these involve enforcing a future story state, the focus is entirely on either actually or functionally denying player actions unless and until they conform to whatever you already pre-approved.
Both together: "The villagers refuse to talk to you" (because you want the players to move on), "you can't leave town by wagon, because all the horses are sick with horse-plague, and you can't walk out because the horse-plague can be carried by humans so you're under quarantine for the next month...but a sailing ship can get you out in two months so you won't have to quarantine when you arrive!", "raise dead fails, the murder victim doesn't want to be revived, you don't know why." Etc. Nixing reasonable attempted actions
in order to enforce a sequence of events.
Like I said, the two are very similar. I see the key difference as, again, one of "adventure design" or "plot" vs "action resolution" or "adjudication." Macro vs micro, one might say. Micromanaging is one way to achieve macro control, but hardly the only way. If one is ensuring a specific sequence of enforced events, regardless of player input, the one is railroading. If one is rejecting player inputs until those inputs match the pre-approved list, one is engaging in MMI. One need not have a pre-approval list in order to have a specific sequence of enforced events. One need not have a specific sequence of enforced events in order to have a pre-approval list.
You said a lot more here, but this particular sentence seems to align with my conception of your position, but everytime I claim that's what people are saying I get told it's not. If normal MMI is asking permission and directly get told yes or no, then all D&D play can be categorized as normal MMI. It's tautological to that definition and how D&D is played. That's fine, but if that's all MMI is, then it doesn't come across as an actual negative.
You have incorrectly understood.
My issue is not "you should talk with your DM and get them on board." That, as you say, is a fundamental part of playing a game with a player who possesses adjudication powers.
I am not talking about all possible forms of "DM approval." I am talking about tables where the DM will reject out of hand
anything that does not conform to preconceived notions, sometimes literally manifesting as an actual list of actions that have pre-approval and rejecting everything else, sometimes metaphorically in the form of (to use my earlier terms) "curmudgeonly" or "miserly" DMing. I know the Rustic Hospitality problem has been cited as perhaps overused in the discussion here, but "miserly" is exactly how I would view the adjudication of what the players got from its use in that case, which was then compounded by separate, non-MMI railroading (via denial of any ability to actually respond to being ambushed, despite the PCs having plenty of opportunity to keep watch and detect such an ambush in advance.)
Yes, DM approval is a thing, and exists for a good reason. MMI is when that approval is granted only begrudgingly, with inappropriate and often capricious or inconsistent limitations or denials. Surely you can agree that "some things should be subject to the approval of a real human" is distinct from "if what you want to do isn't on my mental list/already written down, I won't let you do it." Because, again, it can also be caused by enforcing draconian and foolishly restrictive rules, not just capricious DMing. The latter is just more of a problem because it is way harder to criticize an invisible list than a visible one.