Anyone can spend 2min googling "wrongful convictions" and the number of people found innocent by the judicial process to see why.
Not to mention the whole reason why "lynch mob" style justice is a terrible idea (it's a highly emotional process which gets it wrong wayyy more often than courts/police, the punishments are often way out of proportion to the crimes, no due process, etc, etc).
This is just as good an argument against any form of law enforcement though. Don't imprison people because you will undoubtedly also target innocent people. Mind you, I'm not pro capital punishment, but "don't do it, because you will also hit innocent people" isn't a good argument against it.
In San Francisco you can walk down market and see people dealing drugs, but would never rob you. The former because the police have a no persecute policy, the latter because they have a no tolerance policy.
In places like Argentina or brazil, the mere sight of people like this means 100% guaranteed robbery. I believe americans visiting the wrong neighborhood in Argentina or Brazil would have deeply false sense of security, which is why they are prime targets.
Pretty much everybody prefers that over total gang land chaos. Don't forget that "living in Scandinavia" isn't the usual alternative.
Some people genuinely want to live in the Judge Dredd universe..