Why is plagiarism so harshly punished?
One key difference between plagiarism and violence is that plagiarism is a specifically academic offense, while violence is already handled by the legal system.  If a violent incident is sufficiently serious, it can and should be dealt with in court.  This means university rules only need to deal with cases in which the people involved prefer not to take legal action, and they can leave more serious cases to the legal system.  In particular, the university rules are typically geared towards the less serious end, since those are the only cases they expect to handle.  (If a student or colleague punched me in the face, I would press charges in court, rather than relying on the university to administer justice.  By contrast, if two athletes got worked up and started fighting during a high-stakes game, it's possible that neither one would consider the incident worthy of legal action.)

Plagiarism is not always punished severely: a first offense or minor case may be treated leniently.  However, the rules allow severe punishments because there are no courts to fall back on.  By contrast, universities don't need to have special rules for how to punish a truly dangerous student.