I think it goes both when when it comes to employees/players and employers/schools. Yes, you want the loyalty from the players, but what about the promises that are being made to these players? How often are the players told "we want you at our school...you are going to be a big part of our team next year and the following years", only to get there and ride the pine? Similarly, how often do companies talk themselves up during interviews with potential hires, and then that person finds themselves working weekends and/or longer hours than expected?
The people in charge are not just coaches, but are also salesmen. They will say whatever they need to say to get the sale/recruit. With that comes many a broken promises, so you can't fault a player for going elsewhere if he feels slighted.