learn
Without firm and rigourusly enforced rules and regulations and strictly written laws this kind of abuse is inevitable.
The stanford prison experiment highlights what happens when you give people power over other people with little control http://en.wikipedia.org/wiki/Stanford_prison_experiment
I'm sure a number of officers join intending to be good upstanding people but the job itself lends to corruption and where you have law enforcment units that are allowed to do what they want more or less (like that old unit that got dispanded) or a whole bank of laws that are supposed to be "interpreted" (as the last government was keen on) then these kinds of things are going to happen more and more.
Especially when the enforcment of rules and the allocation of punishment is so lacks (they know they can get away with it, even on the off chance they do get caught it'll probably get brushed under the carpet.)