Officers are trained to not take ANY risks with their safety...ever, when it comes to someone holding a potential weapon.    

The overwhelming response identified in police reports as to why an officer shot is "I feared for my safety".  Society doesn't expect cops to catch bullets, but rarely is the rational, "I feared for the public's safety".  We expect cops will put themselves in harms way to uphold the safety of the citizenry, even the man holding the knife.

Shooting the subject more often then not doesn't serve to increase anyone's safety, other then the officer.   

More often then not a violent confrontation is initiated by immediately drawing guns and every cop there screaming commands at a confused and agitated individual.  It's a self-fulfilling prophecy when the subject then responds to this assaultive act with violence.