I'm... I'm pretty sure this isn't how the law works. When someone dies because of something you did, you don't get off scott free because of gross negligence.
It would have been more surprising if they were punished, frankly. Even if they had been brought to trial, there's no guarantee. Even judges come up with fantastically tortured logic to excuse abuse and mistreatment of prisoners. There's entirely too much emphasis on what the guards intended or what their state of mind was and entirely too much deference to the claims of the guardswhich, given the track record, should be given about as much credence as the testimony of police officers (i.e. almost none except where it can be corroborated by other evidence).