What does Johnny do that is surprising? Why didn't Dally react?

Johnny steps in and stops Dally from harassing Cherry. Dally doesn't react because he loves Johnny like a brother.... Johnny is also "the gang's pet." Everyone looks out for him.

Dallas scowled for a second. If it had been me, or Two-Bit, or Soda or Steve, or anyone but Johnny, Dally would have flattened him without a moment's hesitation. You just didn't tell Dally Winston what to do. One time, in a dime store, a guy told him to move over at the candy counter. Dally had turned around and belted him so hard it knocked a tooth loose. A complete stranger, too. But Johnny was the gang's pet, and Dally just couldn't hit him. He was Dally's pet, too.