The definition of a weapon is something that is used to coerce or effect someone else... legally it is a weapon, like leaflets can be used as a weapon, or tannoy systems can be aswel... And as they are weapons they should only be used in lasst resort, and when there is no other means...
The problem we have is the perspective... we cannot tell what the policeman who used the weapon's thought process was as he used it. He may be following a procedure that is stated in his rulebook, or he may have been in a similar situation where he wished he had used it, so when faced with the same problem this time he did use it. Its very easy to criticise when you are seeing things from a different veiwpoint... then again he could just be a trigger happy sadistic power-crazed fuck... we just don't know.
It is a weapon though, and should be treated as such, last resort only, and when the user honestly beleives that it needs to be used.
It's a similar situation to when you hear about soldiers who shoot civilians driving towards them... Usually it is the result of the civilian driver not stopping or slowing down as they approach a checkpoint, and ignore the warnings... instantly the soldier is going to think it's a vehicle-bound suicide IED, so shoots... the military solely shoot to kill, as shooting to wound is in-humane, wo therefore it is not taught. This action results in a civilian dying and a lot of bad press about NATO/ISAF forces killing civilians... Its perception... the soldier did comletely the right thing, his job, but gets berated by the media for something that is totally perception based...
The taser incidents are of a similar nature... Its process driven, in what is an uncalm situation where decision making is difficult. A decision is made and acted on, but the consequences are unknown at the time.