Why doesn't Holden throw the snowball?
When Holden makes the snowball from his window, he is very nervous because Jane Gallagher is out on a date with Ward Stradlater, his roommate who is very slick, handsome and has a reputation with the girls.
Holden is holding his feelings in about Jane and Stradlater, and he is using the snowball to help him cope with his fear that Stradlater was going to force Jane to do sexual things with him against her will. Holden holds Jane on a pedestal, she is his dream girl.
"The snow was very good for packing. I didn't throw it any anything, though. I started to throw it. At a car that was parked across the street. But I changed my mind. The car looked so nice and white. Then I started to throw it at a hydrant, but that looked too nice and white, too. Finally, I didn't throw it at anything. All I did was close the window and walk around the room with the snowball, packing it harder." (Salinger)
This is also an illustration of Holden's inability to make a decision, he is a deep thinker, even when it comes to picking a target for his snowball. He can't make up his mind about anything, not even what to throw a snowball at.
And finally, he takes the snowball on the bus into town, and the driver makes him throw it out. He wasn't going to throw it anything.
"I told him I wasn't going to chuck it at anybody, but he wouldn't believe me. People never believe you." (Salinger)
Holden actually gets attached to the snowball and doesn't want to part with it, but the bus driver makes him throw it out.