Violence in sport is normalised within society, just like violence on the road. That's the environment within which the police/CPS/courts operate, and that's why the Ferguson case was so unusual.
Fundamentally, when you put people into this scenario where there's an acceptable level of violence built into the rules of the game, and then there's a line, and that line can be crossed and endanger people with or without intent, it's really complicated. I don't know if there's a name for what the offender does in this case, but there's a whole list of named actions that are banned and dangerous, and we know about because they happen - from spear tackles, clotheslines, helmet to helmet hits, 2 footers, the list goes on. And all of those offenses can be the result of a player trying to do something legal and doing it poorly. The language used to describe intent is often violent or martial as well, but it doesn't necessarily signify a real threat. "He's a violent runner" is one of the highest compliments you can pay a running back in American Football - "punishing" the tackler, but there's no malice there.
Generally the rules are about protecting "defenceless" players, and they have evolved over time as people get hurt, and the law sits back and lets sport get on with it. I don't think that's right, but it becomes very hard to judge, except in real standout cases, and unusual cases make bad law. And then you have to think about combat sports, and that's a whole 'nother level of complexity.
