Their goal was not to punish the wrong doers, their goal was to exact vengeance for those that wished it.
That's true, someone has to
wish for a Vengeance demon to act.
What if the kidnapper hurt and abused the child? Do you think she would have cursed him?
The child's parents, or the child himself/herself would have to wish for some kind of punishment, and that (the wish) sure happens in those cases - but kidnap, abuse, murder are already punished by human laws.
Wronged women, children who
feel neglected (like Dawn in
Older and Far Away) or a girl who thinks that another girl is messing with her life and wants her gone (Cordelia, Buffy), on the other hand, don't get to be “avenged” by law, so they need a little help from a Vengeance demon.
If Vengeance demons granted wishes made by people who were victims of crimes, they would be kind of heroes, which I think it's not the case.
CORDELIA : Yeah, I swear! I
wish Buffy Summers had never come to Sunnydale.
DAWN: Yeah. Kinda. I-I mean, yes. People keep... people have a tendency to go away... and, I miss them. And sometimes... I
wish I could just make them stop. Going away.
YOUNG WOMAN: They laughed and they laughed and I cried and they laughed. And I yelled, "Just once, I
wish you could all feel what it's like to have your hearts ripped out. Just once. I wish..." and it came.