speculation
cops have incentive to shoot in ambiguous scenarios involving perps because
(1) involuntary paid time off after all shootings
(2) shooting is regarded as ritual, increases fraternal bonding and trust with fellow officers
it would be interesting to know if there was a correlation between cop shootings and cop promotions
some random (and, allegedly, rare) statistics on cop shootings of perps (2011):
http://jimfishertruecrime.blogspot.com/2012/01/police-involved-shootings-2011-annual.html