Because the direction the players went was a choice they made. And by having it always go to the outcome the GM had planned, you are stripping that choice of any value.
As I've already asked, why is this any different from choosing the colour of the PCs' cloaks - this latter is a choice the players make, and it almost never makes a difference to anything that happens next. So why isn't that railroading?
I am assuming this is a situation where the players believe their choice is driving where they end up.
Well of course the choice will drive where they end up - if you sail east across the Mediterranean you're going to get closer to Dalmatia than Ireland. But that doesn't mean it will determine what challenges the PCs confront.
In most campaigns choosing which direction to go is not the same kind of thing as choosing which color cloak to have (especially if that decision is what is supposed to determine what you encounter or where you end up: and you reach those things regardless of the direction you choose).
You're assuming that
choosing where to go should be different from
choosing what colour cloak to wear. But why? That's just convention, inherited from dungeon crawls and hex crawls, and to treat it as more than that is just begging the question! I mean, it seems pretty obvious to me that in a campaign in which the GM treats the direction of travel as mere flavour, the players choosing which direction to go is
not much different from choosing the colour of cloak to wear; and is
not supposed to determine what encounter takes place!
in certain games maybe that is just flavor but those are definitely edge cases
If the GM is telling them what he is doing, like I said, I don't have a problem with it. It is still railroading, but at least people have bought into the idea. But there isn't any reason for me to assume this isn't secret.
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In most situations, this sort of action by the GM will be regarded as railroading. If there is some mitigating reason why it isn't the poster can include that detail in the example. Otherwise I just have to go with some baseline assumptions.
I can't tell if you're saying that it is always railroading but you don't think it's wrong, if you're saying that it isn't railroading because of some "mitigating reason". (And I don't really get your use of normative language ("mitigating") here - your preferences aren't normative for RPGing in general.)
And I don't understand your assumptions. If someone is presenting this sort of thing and saying it is
not railroading, why would assume they're talking about a game where it
would be railroading? Isn't it more likely that they're imaging a game where it wouldn't be, because the table doesn't assume that choices about geography and direction of travel are all that important?
I mean, there's nothing special about imaginary geography that means it
has to matter in RPGing. In The Dying Earth RPG your clothes matter more than where you go. In Agon 2nd ed the GM just tells you, at the start of each session, which island your ship has been brought to. What matters is which gods you upset or propitiate.
Now if the GM is lying that's a different thing, but why are we assuming the GM is pretending the choice matters as more than just a bit of colour?