Why does Tommo peaceful not get a court martial?

There are only two people who can give evidence against Charlie, and they are Tommo and the Sergeant. Hanley tells Charlie that it will be a court-martial for him if he refuses to charge, along with death by firing squad. Charlie, however, remains steadfast and says that he needs to stay with his brother Tommo, who has been wounded. Tommo isn't court-martialed because he was unable to charge.