Earlier quoted context omitted.
Because sitting on benches quickly turns into living on benches. Then the drug dealers move in, because there's a ready customer base of drug users . Then the productive members of society move out.
So basically the problem is that productive members of society and drug dealers are incapable of existing near to each other? Why is that? I pass by some drug dealers sometimes on the way to work. I don't see the problem. Occasionally I get asked if I want to buy some drugs. I don't want to buy drugs so nothing else happens.
Drug dealers are criminals. Criminals commit other crimes and attract other criminals and crime. For example, rival drug dealers who want to take their spot and use violence.