Didds - the RFU has a legal document, which sets out what you legally accept when you step onto the pitch. I have not read all of it, but it sets out that some action, even if illegal in Rugby, are no longer illegal in the eyes of the law.
If you think this is strange - think about boxing - two people punching in a way that if it happened in the street, would be criminal behaviour.
Flipflop, I haven't seen the documetn you mention, but I imagine that it takes as its starting point the fact that people playing contact sports consent to a degree of contact. It is the extent of that degree that is important, though, and that extent is probably itself informed by the laws of the game in question.
You mention boxing. It is fully in accordance with the laws of boxing to punch your opponent in the face as hard as you can. By stepping int the ring, you consent to that degree of violence being inflicted upon you, and you very reasonably expect to be free from prosecution in the event that your own punch hurts your opponent. But if you take off you glove and hit just as hard, I suggest your immunity disappears because the opponent has not consented to being hit with an unmuffled fist. Indeed, I suspect bare-knuckle fighting is not a legal sport in the UK.
In rugby, one consents to being assaulted by an opponent in a prescribed manner; shoulder first, followed by a wrap. If the tackler gets it wrong and executes a shoulder charge instead, with or without intent, that is a deviation that is not acceptable in the laws of the game, but is not sufficiently wide of the accepted mark to make the action actionable (as it were). But if the tackler puts in a studs-up drop-kick, that is probably actionable as being too wide of the degree of violence accepted by stepping onto the pitch. Ditto the haymaker.
As to whether such unacceptable assaults should result in more police intervention, I leave that entirely to the aggrieved party. The offender has rendered himself liable to prosecution by his illegal action. If the aggrieved party decides that no real purpose is served by lodging a police complaint, I am not going to argue him out of that position. But equally, if he feels that the offender needs to be brought to justice, I'm not going to argue him out of that either.