Why do spectators often wave white towels in NBA games now?

This is to interfere with the attack of the visiting team. When the visiting team made free throws, the towels and sticks behind the blue board waved like snowflakes all over the sky. In the NBA, there are other ways to interfere with the visiting team's attack. For example, the audience of the Spurs home team's stadium is farther away from the stadium than other stadiums, which makes the visiting team feel that the distance of the blue basket is farther than it looks. The famous Madison Garden Stadium is even more worrying for the visiting team, and strange sounds that make the visiting team tremble are heard from time to time.