A sporting arena is a public place where a lot can go wrong. So people who attend an event need to be made to behave as they would in any shopping mall or other public space. What we are seeing is making allowances for them that then threaten the well being of the other people attending. Rooting for a side is not an invitation to violence.
Fans should not be allowed to behave however they want because sporting events are a family event and can be open to everyone. Parents often bring their children to sporting events, so others have to keep this in mind. Also, overly rowdy fans can ruin the experience of fans who simply want to root for their team. If fans are allowed to do whatever they want, there has to be an age limit on tickets as well as a disclaimer warning others about it.