This is so simple I'm not going to spend more than a few lines on it.
Everyone who's wasting words about the First Amendment and "privacy" (whatever that means) is spinning their wheels. "Congress shall make no law. . . ."
A corollary of this obvious prohibition against government, not NBA, action, is that the NBA can make any rules it wishes. . . .
And when Sterling signed on, he agreed to those rules. Yes, even the "75%" rule.
Sterling entered into a contract--a voluntary agreement to terms--which allowed the NBA Commissioner to do what he did.