The problem is that the powers vested in the Commissioner are so broadly defined that there is no way Sterling can prove that Silver exceeded his authority. The NBA Constitution says that the Commissioner can punish an owner for any actions not covered by any of the listed punishments, and which the Commissioner believes was not in the best interests of the NBA. The sole judge of that is the Commissioner himself. Silver can say that because of Sterlings actions the NBA was faced with loss of revenue, loss of the playoffs, and player dissention. And that those actions were not in the best interests of the NBA. How do you prove him wrong? You can't, so the fine and the banning will be upheld in court.
Sounds like you know far more than I do about the ownership agreements, I’ll happily cede to your expertise on this one.