One of my greatest dissatisfactions with academe is the kind of underlying encouragement to see the job as a calling, to see one's commitment to a job as a "marriage" or becoming part of a "family," because I think this is part of the way that academe abuses its practitioners.
Like liz ferszt in the comments section at Dr. Crazy's, I've seen this from both sides; there's no question that having a faculty member come in for a year and then leave is disruptive, especially if the department is small, the budget is tight, and the powers that be would rather fund yet another Assistant to the Associate Sub-Head in Charge of Administrivia than shell out for a faculty line to replace the departing person. The fear of having junior faculty move eventually even causes some haunting fears at hiring time: "If we hire so and so, will she stay?" This can't, and doesn't, enter into the decision-making process, but it's not an unrealistic idea.
That said, I'd side with the junior faculty on this one. Although we'd like to deny it, working at a university is like a marriage, albeit an old-fashioned and unequal one with all sorts of sexist overtones. We (academics) are the dewy, eager prospective brides who want the institution to "marry" us by offering a tenure-track position. We don't want to be the ones good enough to teach courses for a university but not good enough to hire when a t-t position comes available. Even as t-t faculty, all the power is implicitly on one side: "We chose you out of hundreds of applicants. If you behave yourself, publish like mad, and seem grateful enough, at the end of six years we may make this marriage permanent--unless, of course, we decide we'd like someone newer and more exciting." It seems to me that the increasing trend for junior t-t faculty to explore their options on the job market is just an attempt to even up this power differential.
I don't think that institutions mean to abuse this power. Departments hire candidates because they want to see them stay, of course. But whatever it may promise in the courtship process (and there's that metaphor again), however much it might want to behave ethically and in the best interests of the faculty member, an institution will always place its own interests first. Always. That's the nature of an institution. A faculty member who forgets this, who places the institution's interests ahead of her own or at least refuses to acknowledge that the two are different, does so at her peril.