Why did Ponyboy say, "An' you can shut your trap, Johnny Cade, 'cause we all know you ain't wanted at home, either."?

Two-bit says that Ponyboy is all wrong about how Darry feels, and Johnny says he's surprised that Ponyboy feels that way, since he thought they got along. Ponyboy feels stupid because he knows he has it good compared to the home Johnny lives in, where his parents beat him up. So Ponyboy lashes out at Johnny, saying, "An' you can shut your trap, Johnny Cade, 'cause we all know you ain't wanted at home, either. And you can't blame them." Johnny winces, and Two-bit defends Johnny.