Why do bouncers always get away with Murder?

It seems every month there is a new story about a patron being chocked to death or having the pedigree performed on them on a concrete sidewalk. But almost all of the time the bouncers are acquitted and almost never end up doing time. Why don't prosecutors perform their duty and put these roid monkeys in a cell where they belong? As far as I'm concerned if patrons continue to be assaulted, maimed and murdered the steroid junkies shouldn't be surprised when someone pulls a gun and blasts them in their face. Why won't civil authorities protect patrons? Why do they coddle the muscle heads?