At least if you're playing a traditional RPG - D&D, Rifts, Shadowrun, etc - one of the highest responsibilities of the GM is to be fair. That means being a neutral arbiter of conflicts, but it also means not changing the world based on GM knowledge that the NPCs don't have.
Just as it isn't fair to block a player - "You want there to be a haberdasher, which means you must be up to something, so no there isn't a haberdasher," - it's equally not fair to intentionally contrive the world to enable the player - "You want there to be a haberdasher, which means you have something awesome in mind, so there's totally a haberdasher there for you." Those are both demonstrating bias, and it doesn't really matter whether it's working for or against the player, because either way is a violation of the GM's duty.
As a player, I might want there to be a haberdasher, but I don't want there to be a haberdasher only because I want it to be there. That's inauthentic, and it's pandering, and I don't see any point in playing that game. Of course, the GM could lie and change the reality to block or enable my idea anyway (and it would be super easy, if nothing it written down), but that's why it's so important that the player can trust the GM to play fairly. Without trust, the whole game falls apart.
I won't say that trust is a given, but if you lack trust in your GM, you have a problem larger than the scope of FF, rail roading, etc. You'd basically have a GM you can't/shouldn't play with.
Therefore, let's bound the discussion to a GM you like and trust. He's trying to be fair and you trust him to do his job within the style he runs his game.
In your hat shop example. You've given a "no the GM shouldn't let there be a hat shop" and a "yes he should let there be a hat shop" reason.
That's pretty much a road block. The GM needs to answer the question of "is there a hat shop" or not, so in the presence of two conflicting arguments of relatively equal merit, he still has to make a decision for which those reasons don't resolve it for him one way or the other.
If the GM has truly mapped the entire town, he can simply defer to the map. It's not his decision at all, other than the choice he made earlier during the map making process to put one in there or not. But then, that might have been an oversight, as he made a mistake and didn't think of hat shops as a possibility. So now we're second guessing his prior decision making skills, which might have been a random town generator that simply lacked the option for a hat shop, in which case, no town in his world has a hat shop, despite it being a reasonable possibility.
I would say this, from what I can tell online, [MENTION=2]Piratecat[/MENTION] is a famously excellent GM who improvises a lot. Thus the quote "You want there to be a haberdasher, which means you have something awesome in mind, so there's totally a haberdasher there for you." would not be a violation of his duty, because it is exactly what his players should expect from him to make his game excellent (for those who enjoy his style of game).*
Thus, a value judgement of "saying Yes to enable awesome is Wrong" as per your examples can be invalidated as being subjective, based on the kind of GM/game style being run.
I do think that "being fair" is generally valued across all players, so the argument for saying "no" with the intention to always block the players is generally a bad thing because it violates the fairness (and thus Trust) guideline.
So what's GM to do?
I suspect the guidance for a GM to the question of "Is there a haberdasher nearby" is to answer Maybe, or Yes in most cases.
Yes is for GMs who favor an Improv style where there rule in improv is to say "yes, and..."
The Maybe response is to systematize the answer. Either let the map decide, if you assert the map was fairly generated (allowed for the possibility of reasonable town contents like hat shops), or to roll a dice (50:50 chance for yes/no). and go with that. You'd be removing the GM as a human from the equation within reason for purposes of deciding the answer.
*I have never met or played with PirateCat, nor do I wish to put words in his mouth, but I have seen enough of his posts to know he seems to have an improvisational style and he is well regarded on EN World for his GMing skills, among other abilities. Hopefully my mention of him will drag him over here from the GumShoe thread to lend his wisdom to the discussion.
