I think the school really did try to help the kids to understand what the problem was, from what you describe. As other posters have mentioned, sometimes the group dynamic between even a few kids can affect the whole. If teachers were dealing with kids hitting and hurting each other during a specific activity, they are obligated to ensure that the activity isn't allowed to happen.
Also consider that for insurance purposes, this is a known liability. The school, knowing that rough play and fighting was happening, was under every obligation to protect itself by banning that activity for the present. If parents found out there were problems previously but their child was hurt, they may very well consider suing, and with due cause-- a known problem was allowed to continue.
It's too bad that it's come to this, but perhaps next year there will be enough peer pressure from the other students who didn't participate in the aggression and don't want to have to lose soccer again. They will remember that the school *will* take away a beloved activity if people aren't staying safe.