The premise of your question (as you conveyed it) is not precisely correct. Two of the officers involved in the Rodney King incident, Laurence Powell and Sergeant Stacey Koon, *were* eventually convicted on Federal criminal charges of having violated Rodney King's civil rights.
1. Two types of "non-lethal" weapons (in other words, not *deadly weapons* as a handgun would be considered) were employed in the Rodney King event: the Taser and the PR-24 baton (a specialized sort of nightstick, evolving from an old martial arts weapon, specifically developed for law enforcement purposes for *defense* and restraint purposes.) In order for an officer to carry a PR-24 baton most police departments require mandatory training, provided by the device's manufacturer (Monadnock.) All the involved officers were indeed trained in its proper use and, to the degree that the PR-24s were employed, the officers used them as they were trained to do, impacting King in strategic places to effect his inability to fight back and his ultimate compliance with verbal commands; HOWEVER, instead of focusing on King's actual compliance, some of the officers saw King’s continued “movements” as justification for unrelenting aggression. Other officers stood back and watched (e.g., California Highway Patrol officers) which, if it was really an objective to get King under control and in custody, they could have easily assisted in effecting this end, thereby abbreviating the ongoing beatings.

2. King was clearly under the influence of drugs and/or alcohol. Such people often have an incredibly high tolerance for pain and they often don't go down after a Taser (electronic) shock or a strategic blow with a baton such as a sober person would. Again, the officers should have (and clearly would have) known this from both training and experience. The obvious solution was to simply pile on and force King into a position (by multiple officers holding him so that he could no longer resist arrest) whereby he could be handcuffed. This was not done, for one reason, because officers from multiple agencies were involved (LAPD officers were the primary officers attempting to arrest King) and as an unspoken rule, officers from one agency tend not to interfere with the actions of other officers, in other words, "blue solidarity" of a sort, right or wrong.

3. I'm a professional 25-year career law enforcement officer, (now retired). I can assure anyone that with one other assisting officer, I could have and would have (due to my training in both physical procedures of arrest and in understanding the lawful use of force restrictions) arrested King and had him handcuffed in very short order. This is not what happened -- the LAPD officers in particular chose to make sport of the King arrest, AND they were also *psychologically* driven to initiate *punishment* and aggression upon King (not a legal prerogative of law enforcement officers.) This latter phenomenon often occurs subsequent to a police chase (the adrenalin rush actually causes *”tunnel vision” of thinking and focus) and the officers must be *trained* to control