The NBA blocked them with the consent of the players association. the NBA cannot unilaterally make that change. And the players agreed, because as we've seen year after year, athletes are terrible at handling their business. They continue to get railed in CBA negotiations and leads them to make poor hiring decisions (Tony Clark, DeMo Smith, this new chick for the NBPA) who each get beat up badly at the negotiating table. This is one of the places the players should be giving the agents a bigger voice at.
Also, the Commissioner should be a neutral 3rd party, paid a % of the basketball revenue, but that's a separate issue.