Because in some rail yard somewhere lurks some really decked out train cars, you want to prevent a neighborhood plagued with gang violence from protecting their community?
If you really want to stop gang violence, people in poverty need opportunity and investment. Preventing gang members from painting things does nothing to solve the problem of violence, that's an absurd conclusion.
The theory you are presenting, of monocausal criminality due to economic conditions, has been analyzed thoroughly and rejected decisively by the field of criminology for decades, now. It's easy to see why: we can compare impoverished groups both nationally and globally and discover that there is simply no causal, consistent relationship between poverty and violent criminality.
To be sure, economics can play a role and become a factor in motivating criminal behavior, but violent criminality is ultimately a choice that people make, not a direct consequence of poverty. Indeed, many groups suffering tremendous poverty exhibit very low rates of violent crime, and likewise, Suge Knight exists.