Tournament adventures are one of the few times when railroading and linear plots are not only not a bad thing, but generally considered a good thing. As noted above, adding non-linear elements means more work for the designer and more work for the DM, for a net gain of no extra fun for the players.
And, in fact, if the game is too open, it's entirely likely that the PCs may spend too much of the allotted four hours going around in circles, and not get anywhere in the plot. That's no big deal in a campaign, where you can adjurn until next week, but it's death to a tournament adventure.