Just as there are programs which seem not to prefer their own, there are those which seem only to accept grads of the affiliated medical school. One only needs to look at web sites of various programs to see that many programs are "in-bred", that is the residents often went to med school and even undergraduate at the same campus.
However, I doubt any school has a formal policy of restricting its own graduates or only accepting homegrowns. Just a history of one or the other which can easily be ascertained.
The reason behind a decision not to take many homegrowns is often given as a desire to broaden experiences, bring in information and people from different areas of the country. Too many people training at the same place tend to perpetuate a "practice model" which isn't necessarily the best - faculty, residents and students from all over tend to bring fresh ideas to a program.