I honestly don't know why -some- superstars don't forego their nba salary completely.
Lebron has a 500,000,000$ (half a Billion. With a B) endorsement contract with his shoe manufacturer. He earns an order of magnitude more from them than from the cavaliers.
He could drop his NBA salary down to 1$/yr and still have more money that he could spend. And it would allow the team to go after anyone they wanted.
There are several NBA players in that position (though not quite as much as Lebron's endorsement contracts)
Now Lonzo Ball comes along and demands endorsement contracts even higher (3 billion? c'mon) - but even if he only gets 1 billion (which isnt unlikely, eventually, just not yet) even a 30 million/year contract from the NBA is small potatoes comparatively.
And if a player forgoes their salary, and his team acquires the best free agents with their cap room, and that team wins more titles, the superstar player's endorsement value goes up even more than their salary that they gave up.
Just rambling....