Its the man in the environment.  You can't have somone not willing participate.  What sometimes happens however is through week leadership usually at the squad level somone gets in that is of less than sterling character.  This sometimes over years can lead to the best people leaving the squad either through choice or retirement.  This in turn leads to members of the squad helping to choose their replacements.  As this goes on those with less than sterling character get in.  This is where the supervisor's failings come in.  Letting the troops run the unit instead of the other way around.  

This can be exacerbated in specialized units as supervisors are sometimes promoted from within. They are attuned to the unit culture and not that of the agency at large. The results are disasterous when you already have a culture that will tolerate less than proper conduct. In large agencies a promotion in normal squads or units always means transfer.  You are not supervising the people who were your immediate peers.  

In most cases I think you will find that most Police agencies put their best and brightest in special units. They supervise them well and are highly intollerant of wrongdoing.  Hope this answers your questions to a degree.