the Jester
Legend
Or another way to look at it: 'Does anyone at the table care?'
If everyone in the game is down with going to planet Y and doing the mission thing and you're all having fun, it doesn't need a label. It's functional, it's fun, it's play.
And this is an excellent point. As several folks have pointed out, there is a spectrum here, not two distinct states. (From railroad to rowboat, the extreme and negative ends, through story-based and sandbox play styles to some mushed-up half-and-half in the middle.)
I think this is open to some legitimate debate. Saying "you can't find X no matter what you do" is certainly a railroading technique.
But given the full context of how Gygax designed his campaigns (with dungeons that constantly shifted and changed when you weren't looking at them), this particular application looks more like "they can't find it because it doesn't exist yet; if some sort of explanation is later demanded, here's a retcon you can use".
And failing to find something because it doesn't exist wouldn't be railroading.
First of all, RC, you're covered.

Second, this is a very important point. The dungeons of early D&D were not static. Passages appeared and disappeared, populations came and vanished almost magically, doors stuck for adventurers but not monsters, all monsters could see in the dark- all IIRC, but I don't think I'm far off. Early dungeons were almost alive in themselves, constantly mutating.
(As an aside, I think the mythology surrounding Torog in 4e is a great step towards this type of dungeon.)
All that said...
Does it limit a PCs legitimate choices in a given scenario? (IE Search the area, find the dungeon)?
Does it create a bottle-neck for the adventure the DM has prepped over what the PCs have chosen to do?
Is it done for an out-of-game, rather than in-game reason?
Is it explained in the world via a handwave that cannot be detected, dispelled, or otherwise affected by the PCs?
How, exactly, is it any different from telling your PCs "You can't go south, I don't have an adventure planned"? Shouldn't the DM have to "wing it" because that's part of the beauty of sandbox play?
Unless you make some kind of assumption that justifies the whole "Whoops, how did I miss that?" thing, I would agree that this is railroading.
"Can" and "should" are what are in question here. The DM does a lot of work, is it right that the players should expect him to do more just because they want to go in X direction?
Sure, the DM can wing it, but I highly doubt it's going to be as good as what he had planned.
Which begs the question: if the choices of the PCs make the game sub-par, should the DM indulge them?
Which further begs: if players decisions should always be indulged, what is the point in creating any content for them, if the game is always going to give them what they want?
Well, the dm can feel free to say either, "OK, we'll have to resume next time" or "Give me a couple of minutes to do some quick prep." Heck, if you have any smokers, you know they're ready for a break any time!

However, one of the arts within the art of sandboxing is the ability to improvise. Most sandbox dms that I have played under seem to do pretty well, and if they don't, the group generally knows that they are leaving the 'cool stuff' when they go in an unexpected direction. It is up to them, though- sometimes they just want to explore something new.
Improvisational dms develop the skill by doing it. While the winged stuff might not be very hot tonight, perhaps by next April it's not bad, and in June and July there could be a couple of mostly improvised sessions that hit a home run. Just like designing encounters, you get better at improvising with practice.
Now, not all dms are good at improvisation, not even all sandbox dms. But let's not just assume that a game is bad because the dm is winging it.
And yes, the dm should 'indulge' the pcs' in-game choices. It simply isn't his place to dictate what they do. As always, YMMV- but to me, that's the whole issue right there.
Re: What's the point of creating content in a sandbox? First, I would guess that most sandbox dms love creating campaign content for its own sake. Second, sandbox content might be for this group, but the next group in the area could easily run across it instead- or as well. Sandboxers get to reuse maps, dugneons, npcs, etc a lot.