I guess I join in the question other's here have raised.  Are police really that stupid that they don't realize their car's video camera is recording their crimes?  Or are they so certain their criminal behavior will never receive an inquiry and be exposed that they do not care?  The latter is quite frightening, but the former is hard to believe. 

Or, the third option of playing the odds that it won't be recorded.

The police car dash cam is the single most malfunctioning piece of equipment ever invented, sold, purchased, or used by man. ;-)