There is some fairness there, although most veterans view it as more of a spectrum than an either/or. If an adventure/campaign was truly 100% sandbox, the GM should quit and join Second City for his improv skills. However, on the plus side, there would be no need for prepping.
Also, part of your argument seems semantical. What if we didn’t say ‘Let’s play D&D tonight.’ What if, instead, we said ‘Let’s play Tomb of Horrors.’ And then, in sandbox style per your implication, the players say ‘We aren’t going in, we’re going to the town and then catch Giant Eagles in the mountains.’ Did they try to sandbox? Yes (for either of our definitions.) Is the opposite railroading? In your definition, yes, but when the agreement was to play Tomb of Horrors, I don’t fully count it as railroading. There should be some structure especially if the GM is spending money and prep-time. And the difference in the situations is calling the game ‘Tomb of Horrors’ instead of ‘D&D.’ -> semantics.
At a smaller scale, though, you are dismissing the sandbox. Yes, we might have to play the dungeon, but old school dungeons usually didn’t have level bosses, didn’t have mini-bosses, had lots of real choices in direction within the dungeon, and didn’t have an end goal to achieve. If there were (or were not as just mentioned) end goals, you could get to the end through path A or path B or path C and by “path” I mean a geographical path, not ‘I RPed bargaining for the macguffin from the townsman instead of stealing it! Now we can open the final door!’ Either of these last two are ‘paths’ to the same waypoint (opening the final door) and that point is necessarily where railroaded parties must go.
In old school, parties could try to kill the dragon to get the Arkansawstone, try stealing it, try buying it, try intimidating it, or ignore the whole encounter as too dangerous because you don’t even need to have the Arkansawstone.
I do concede that most old dungeons gave the GM adventure hooks to entice players with. Stuff like:
[ul]
[li]There is a treasure[/li][li]Save the town-crier[/li][li]Stop the invasion or face the wrath of the king[/li][li]Find out where the spies are and earn an earldom[/li][/ul]
Enforcing these can be viewed as railroading.
…
Once you get to the dead combatant scene at the end of the entry passage of module B1 (this is the first keyed encounter) you can see seven directions to go. None of them is required and they don’t all lead to the same place unless you define ‘clearing the dungeon Mario Brothers-style’ to be the same place (because if you cleared a dungeon you would have been everywhere, so in a sense all the passages led to the same place.)
Here is a thread from another board about dungeon design. Included in the diagrams in the first post is B1 which was titled In Search of the Unknown.
Those are some real choices.