My understanding of economics and game theory lead me to believe that the free market will eventually take care of this "no-poach" cartel (looks like it's already happening). If I recall correctly, this agreement can be reduced to a "prisoner's dilemma" game: the rational decision for a given company is to cheat the agreement or simply not take part in it.
Perhaps economics and game theory will in the long run but here and now the Department of Justice and the class-action plaintiffs are doing a good job taking down the cartel.
