Why do you think Two-Bit was relieved when Ponyboy picked up the glass from the bottle he broke and used to threaten the Socs in The Outsiders?

Two-Bit is relieved that Pony is picking up the broken glass so that it won’t give anyone a flat tire.

In Pony’s world, broken glass is often used as an impromptu weapon.  There are many mentions of it being used this way.  Pony says that his brother Soda “once held off four guys with a busted pop bottle” (Ch. 1).  A person either breaks a bottle to use the jagged edge as a weapon or finds broken glass to use as a kind of knife. 

When Pony and Two-Bit face off against the Socs, Two-Bit takes a pop bottle and breaks it, and then hands off the broken bottle to Pony so that he will have a weapon if it comes to a fight.  Pony tells Cherry he couldn’t use it, but tells Two-Bit a different story. 

"I don't know why I handed you that busted bottle," Two-Bit said, getting to his feet. "You'd never use it."

"Maybe I would have," I said. "Where you headed?"  (Ch. 3)

He is trying to seem tough with Two-Bit.  In reality, Pony is not really the fighting type.  He certainly is not the type to carry or use any kind of weapon.  This is why Johnny is the one who has the weapon and kills Bob in the park.

Much later, when Pony comes back, he faces off against a group of Socs who call him “the guy that killed Bob Sheldon," and does break a bottle.  They leave, surprised, and Two-Bit asks him if he would use it.  Pony doesn’t see why Two-Bit would care. He doesn’t realize that they consider him different from the others.  Two-Bit tells him not to get tough.  When Pony starts to pick up the glass, Two-Bit asks him what he is doing. 

He stared at me for a second, then grinned. "You little sonofagun," he said in a relieved voice. I didn't know what he was talking about, so I just went on picking up the glass from the bottle end and put it in a trash can. I didn't want anyone to get a flat tire. (Ch. 12)

The picking up of the glass demonstrates that Pony is different from the others.  He is straddling the fence between two worlds.  He is able to see broken bottles as weapons, but he also sees them as normal people do, as dangerous to cars and people stepping on them.  Pony is being conscientious in picking up the glass.  Two-Bit is relieved because it means Pony will not get tough.