Answer:
The state charges Johnny with manslaughter due to Johnny being "a greaser". The greasers are known for their violent behaviour, so instead of the state viewing the murder as self-defence, they viewed it as manslaughter. The greaser has always had a bad reputation with the police especially. This is another reason why the state charges Johnny with manslaughter
The socs on the other hand are viewed as the victim and not the aggressor since the socs are known for being wealthy and affluent compared to the greasers. Because of the wealthy reputation of the socs, the state would most likely trust the story of a socs rather than that of Johnny's. Even though the killing of Bob was only an act of self-defence from Johnny, there is no way the state would believe a greaser. In conclusion, the reason why the state charges Johnny with manslaughter is due to the fast of Johnny's reputation as a greaser.