Although anti-poaching agreements could certainly be used to the detriment of employees seeking to further their careers by changing employers, I believe the original reason for such agreements was to prevent one company from interfering with the development plans of another. For example, if Google were to make a significantly above-market offer to the top 20 engineers working on OS X 10.7, it could delay Lion's release by months or even years. If a larger company, like Apple, wanted to put a smaller company permanently out of business, it would just have to make above-market offer to all of the other company's employees.
I must admit, however, that I would be upset if my application for a job was shredded just because of who I currently worked for.
With respect to this case, would the plaintiffs not have to prove that they were not hired for positions for which they were the most qualified candidates?