I'm not condoning what these creepy people do but they are also generally a hindrance to law enforcement and can completely botch things up for the police.

We had one of these incidents near where I live, and they completely destroyed an police investigation. Further the Creep sued both the police and the creep catcher and won.

I know that the police don't have the resources to chase down every thing that is going on, but vigilantism can also do a of damage to the due process that police investigations must follow.