I think the reason most modules tend to rail-road the adventure is because the writers have absolutely no connection to the PCs, or even the adventure up until the beginning of the module. Because the writer cannot in any way judge what the PCs are like, it's hard to come up with a valid reason to get the PCs to want to do the adventure...so they come up with a way to make the PCs do the adventure.
After all, one group might take on the bad guys because it is the right thing to do. Others might do so because they are paid. Or to impress the love-interest. Or simply because the bad guy taunted them. Even if the characters have something valuable to them, the players might not care enough about Mother's Handknitted Orange Cap to go after the thief who took it. (Or maybe a present like that from Mother would indeed inspire a bloodbath!)
Even the same players when playing different characters aren't motivated by the same thing (tho, I'm sure we all know that one player who is always Neutral Greedy no matter what character they play!). Because a module writer cannot tell if a kidnapped cousin, an inherited castle, or the prosmise of a noble title will motivate a particular group or fit in with a campaign, they tend to find a way to force the characters on it. I think it's up to the DM to find the way that fits best (whether that means modifying the content or running it as written) since the DM knows the characters and the game up until the point of the module is introduced.
