What does an agreement between 2 private parties have to do with the Consitution. Sterling agreed to the rules to be an NBA owner and has to abide by the consequences in his franchise agreement.
And if he doesn’t abide by the franchise agreement... the NBA’s recourse is to take him to court. That can take years to work itself out. In the mean time... go to the game. Hire and fire whoever you want... and go on with life as usual.... until a governmental agency restricts you.
It can’t bid any party to perform an illegal act. The civil rights laws weigh on it, for instance.