It comes down to this: unless the injured player can prove that he did nothing to provoke a retaliation response and is willing to file a complaint with the police, the police are very hesitant to intervene on their own.
I can’t remember the year, but there was an incident in the NHL where a hockey player whacked an opponent across the head with his stick. This caused great injury to the attacked player and also outraged hockey fans everywhere. The player who assaulted the other player was fined, suspended and sentenced to one day in jail. There were additional incidents in hockey, but the outcome produced only suspensions and fines. It seems to me that the courts are also very hesitant to hand down sentences that would stop sports violence.
I hear it all the time, “It’s just part of the game.” The old joke told by Rodney Dangerfield, “I went to a fight the other night and a hockey game broke out.” People (fans) love violence in contact sports, like football and hockey. Even during a baseball game, the fans cheer when the benches clear. It’s almost ironic in that the violence that we punish if it happens within the public, some people glorify when the same violence happens on the field or the ice.