clippedgull
Hotdogs, extra onions
Because it's generally not considered in the public interest to get the criminal justice system involved in matters of legitimate sporting contests. By competing in a sport, it's considered that you're by default consenting to a certain amount of physical contact (depending on the sport obviously- clearly this isn't going to apply to, say, badminton). The legal position is that something which ordinarily might be deemed an assault if occurring on the street, would have to go way, way beyond the boundaries of acceptable physical contact and the rules of the game for a prosecution to be pursued. A tackle which results in a serious injury could be through excessive, reckless force, or it could be simply mistimed, an error of judgement. To prove the offence of GBH (given a broken leg, say), the CPS would have to demonstrate that the offender either intended to cause serious injury (difficult to prove) or was subjectively reckless as to whether it occurred (he foresaw the risk but wen on to take it anyway). A player will quite easily be able to say he just mistimed it, and show numerous clips of other unintentional, but mistimed tackles as evidence.
It's likely to be different if the "assault" is something that's not part of normal gameplay itself, for example if you punched or head-butted somebody. Everyone's favourite pundit, Chris Kamara, was done for GBH on the pitch years ago after he ran up to a Shrewsbury player when the game had stopped and punched him in the face, breaking the guy's cheekbone. There was simply no way for him to argue that it was a legitimate footballing challenge that went wrong. From memory, a Barrow player served time for breaking somebody's face in similar circumstances. Tackles though: it's very, very rare.
In the Cooper case, Oxford didn't actually suffer any injury beyond a bit of a bruise or a scratch, I'd have thought, therefore at most you'd be looking at a common assault anyway. Cooper would argue that he had no intention of doing it, and to prosecute him would be a massive waste of public time and money that would, at the absolute most (assuming he doesn't have a long criminal record of violent offences) result in no more than a financial penalty which would be considerably less than the FA could fine him.
Hope that answers your question
Thanks Edna