Those are definitely interesting stats about UCLA, Berkeley and NYU. However, as with all stats, I think there could be many different interpretations.
The most obvious explanation why such a large percentage of graduates from those schools stay within their regions is because that people choose to attend those schools are predisposed to working in the area. In other words, it's a question of cause or effect. I think that many people decide that they want to work in the region, so they apply to the schools that do best in those regions.
Also, I think these stats indicate a tight relation between the schools and those particular regions; and also that those regions have particular appeal. For example, I think it would be commonplace for someone to want to work in LA or New York or San Francisco; and it's no surprise that students from schools in the region choose to remain there. It's less common for people to state that they want to work in the Mid-Atlantic, or the Midwest. Those regions are less-specific, so it makes sense that schools in those regions are no as closely tied.
So, I guess that the conclusion that could be drawn from the stats might indicate that graduates from certain schools choose to stay in certain regions, and may not necessarily indicate that graduates stay because they aren't able to land jobs in other regions. Of course, I do agree that alumni bases will be much weaker in other regions.
I don't think you'll be trapped in those regions, but it probably makes sense to like them ahead of time.