Unless the police are in possession of information not in evidence from the game video, that is a ludicrous decision. The entire hockey world is pretty much united in recognising it as a horrible accident.
Players get hit with skates all the time. Jacob Lauko got a skate in the eye two weeks ago. Unless they've got Petgrave telling Johnson he's going to do him and how, this can only be a decision based on a complete lack of understanding of the dynamics of hockey.