But nobody's saying they shouldn't be punished. Nobody. Apart from some 'liberal do-gooders' who don't actually exist.
It's just that prevention is better than cure. Educating people better and improving the type of people they hang round with is likely to reduce the number of crimes they commit - the more intelligent you are, the less likely you are to think that stabbing someone's a good idea and that covering for your mates when they stab someone is a good idea. It's not rocket science.
Denying this just gives the impression of someone who wants to look tough on crime, rather than someone who wants there to actually be less of it.