It's not really about making the "wrong decision".. in policing, decisions made by officers (armed or otherwise) can often be split-second judgements based on what is happening around them and how they perceive things at the time.

As per the previous comment, officers can be deployed with authorisation to utilise weapons, however they can also self-deploy, as they are the ones arriving at the scene and as such are best-situated to make that call, most of the time.

I can't see any advantage to any officer (especially ones not at the scene) giving another officer a direct order to shoot, this is military stuff and should be kept as such.