I can envisage certain scenarios where a taser might be used on an already restrained subject experiencing a servere, prolonged psychotic episode, if the effect of the taser helped to quell violent thrashing. The act would be in the subject's interest, in that case, since, by definition, he's out of his mind and requires treatment to become stable and end the episode. I'm not an expert, and quite possibly trained mental health professionals would dispute the efficacy of such an action. But to me it seems that in certain circumstances it might be reasonable, effective and ethical.
But violent, non-psychotic, already restrained subjects are another matter altogether. They're not out of their minds, they're just pissed. Their ability to sustain violent thrashing is orders of magnitude less than that of a mental patient, their ability to understand and communicate, orders of magnitude greater. They need to be restrained, but they don't need to be shocked. Eventually, they'll tire themselves out.
The police are not in the business of inflicting pain on anyone. They have the power of arrest, and important power, and they're sworn to exercise it professionally and judiciously. A restrained subject cannot harm a properly trained police officer, and a properly trained police officer should never consider it his duty or right to harm a restrained subject.
Yet they are allowed to use pain compliance techniques to enforce order and arrest suspects. They do it every day, it may not be their "business", but it is a tool they use.
A restrained subject cannot harm a properly trained police officer
This is demonstrably false. Officers have been killed by restrained individuals. Officers are frequently injured by flailing or jack-knifing "restrained" persons and of course, restrained persons have been injured and or died while in restraints.
The FBI and many other agencies collect and study these When this punk and his lawyer get into court they will be faced with reams of studies and experts to show why the taser represents a lower level use of force than other methods like the baton or a choke hold and is therefore reasonable.