The Outsiders is a very violent book. Gang violence, child abuse, stabbings, shootings – these drive the action. The novel explores the impact of living in a place where a teenager can't even walk home by himself and where fear is the predominant emotion, as is the case for recently orphaned Ponyboy Curtis and his friends. While Ponyboy hates the violence and bullying in his neighborhood, he recognizes the positive benefits of friendly sparring between boys, and even "rumbles," so long as weapons aren't used and everybody plays fair. Such activities, he claims, help guys release their endless supplies of energy and pent-up aggression.
According to The Outsiders, fair fights (ones without weapons) aren't really violent; they're healthy avenues for the boys' physical energy, and an opportunity to showcase their physical talents.
Even though Darry slaps Ponyboy, he's not an abusive "parent."