Here is the actual problem which the papers did not present. The camera is an object and cannot charge assault. Assault needs the charge of a victim or a witness. The assaulting officer will not charge himself, and his partner said his interpretation of the events differed from what was on the camera. That leaves the only witness as the victim. This puts the victim in a precarious place. If he starts speaking and revokes his right to silence he endangers his own criminal case by perhaps allowing questions as to why he was in cuffs which he may have to answer after making a claim. So it seems the victim tried to avoid the case and eventually just took the fifth which means that there is no witness or victim left to file the charges. Since the camera cannot be cross examined as is the right of the defendant there can be no charges made by it.
You are right the case seems to be riddled with crappy legal technicalities. The officer may very well know the spot he would be putting the victim in to potentially revoke his right to self incrimination in other charges by speaking as a victim in the assault trial. I know it seems the judge should look at it, but if no one can press charges, or the officer's self defence claim would stand because there would be no one to refute his claims, then they have to drop it. I am not defending the position, just trying to explain what happened.
As for the rehiring that may be a lot more problematic for the officer. He may not be legally guilty of assault, but the state has evidence of very improper actions by him which they could introduce into a legal case for his job back. They have cause to fire him and evidence to present as that cause even without the witness to back it up. Civil trials tend to have much lower standards for admittance of evidence and claims of guilt. Since the state is claiming the video as their reason for termination they should be allowed to enter that in any employment challenge. That is if they are serious about getting rid of the cop which all this publicity will probably make them, but it is not guaranteed. needless to say if the guy remains terminated his options for future hiring become much slimmer given a web search would completely destroy him in a background check.
yes, it is not the best situation, but I am pretty sure the cop knew what was going on and that he could hit the suspect without too much danger to himself due to the conflict of the charges against the victim. He did wait until he was probably in a bay, out of view of most officers, and the only witness against him would have been his partner since the victim's lawyer would probably tell him to plead the fifth. The justice system is skewed to be in favor of the defendant, too bad that seems to be reliable only for law enforcement.
