Because the behavior was disrespectful towards these girls. And the boys should be witness to what that disrespect can lead to. Both the boys and girls in this sad situation should be required to visit, or volunteer at, a battered woman's center so they can see what their behavior, if left unpunished, can lead to.
In January 1999 the UK Government’s Home Office published the results of a survey into domestic violence. It was the biggest ever carried out anywhere in the world and involved more than 10,000 men and women. It was called Study 191 and it stated, quite categorically, that 4.2% of men and 4.2% of women perpetrate the crime of domestic violence. In other words they had discovered that men and women are equally violent.