In many countries, the police has it's own sort of 'corporate culture' that is just not right. But it cán change. There are many examples of how a culture within a policecorps has changed.
It takes time, but eventually the men in uniform will see that abuse of power is unprofessional behaviour that actually harms the police itself and that is in the end counterproductive.
In many cases the policemen themselves will eventually be the first to say that they like joedirt says, should be held to a higher standard than ordinary citizens. That is what professional ethic's eventually requires of them.
Unfortunately, it usually takes a whole lot of incidents to pile-up until the mechanisms of politic's start working properly.
In britain for instance, criticism towards the police is something you increasingly hear after many, many incidents and eventually the politicians can no longer ignore those rumours.
The london metropolitan police is likely to face some tough changes and reforms in the near future. It will take some time, but it will have effects eventually.