How is the NBA draft not an unlawful restraint of trade?

(And all sports drafts.) Players drafted must sign below-market rookie contracts. When I came out of law school I was a free agent. Didn't get many takers, but my starting salary was not already set by a collective of law firms. How is the salary cap not an unfair depression of wages? Why are we protecting billionaires from paying their employees what the market for them will bear? I know the answer to all of my questions: if there's a union, courts will not disturb a collective bargaining agreement, which was just signed by the league and the players association. If I was the NBAPA head the next go-round, I would not budge from the demands of no draft, no cap, and a reduction in regular season games. And the players (average player's salary $9.6M) through the union would fire my ass the next minute.