Mechanically I have created a harder challenge without eliminating the majority of alternate means to solve said challenge... I have also given a reason, the NPC is taking the most direct route.
Imagine we live in a world where DMs may be audited by the Game Master Authority. You've been audited. He's going to ask you to "prove it." Will your declaration of NPC perfect knowledge pass scrutiny?
I realize there's no such thing as the GMA and you are free to do what you want. Some of us are advising, "hey, be careful. that might not be the best practice."
There are always only n ways to circumvent an obstacle... why should any and everything be a possibility at all times?
Things being intentionally limited as "thought of by the NPC, and not the GM" should be a rarity. Moriarty is the rare exception where GM knowledge should be fully available to the NPC. The extreme abuse of this tactic is when every NPC is perfectly prepared for the PCs and thus able to counter the PCs every strategy.
Well I have explained why the PC's cannot use Know(local) in the specific way of finding a shortcut...because the NPC is taking the most direct route. If you equate me setting up one skill as unuseable and an auto-failure with there being only one solution to capture the villain... Well that seems mopre a failing on your part to think of other options as I have certainly listed numerous alternatives earlier in the thread.
I forget who proposed this way back, but technically, if you're playing mapless, the PCs may still need to roll a check to see if they know how to get to point B, and to which route is the best for their needs. That last is important. Regardless of what the GM is doing behind the screen, the PCs don't know that, and are still going to pursue taking they best route they can. They aren't going to deliberately take the worst route.
Which has the ironic twist. The route the PCs take is uncertain. If they fail the skill check miserably, they may completely miss Moriarty's trap by way of incompetence.
That doesn't mean everything should be possible in every situation. Again I point to terrain, hazards and traps... these are all in-combat ways of making the encounter harder, limiting available choices and making the situation more complicated. why are these good and interesting to use in combat, but when I do the same thing in a skill challenge it's "bad design" and objectively wrong?
How about it being bad practice if done all the time. And when someone talks of "I'll do it this way because I'm the GM" it sounds like they're on the way to bad GMing land.
I can't put my finger on it, but there's also a difference in the GM creating starting conditions and hard facts like, there IS a pit trap in the room that the fight takes place versus the NPC knowing the perfect route and planning tricks along the way for the PC.
One is about real world positional facts. The bad guys live in the dungeon of doom. They know the pit trap is there (perhaps Phil fell in?). When the PCs show up, they fight, and use the pit to their advantage.
The other is about an intangible map and information and execution of a chase. There's so many variables there that as a person familiar with the subject of routing, only very special NPCs would be certain of truly having it right. Anybody else (not Moriarty) would be better simulated by die rolls.