The idea is you run from the cops and not go chasing them. By advertising illegal things it's like you're copbaiting in some ways.
Instead of waste 500$ to advertise you can place your self outside LSPD and shout "I am selling heroin" as it's no difference.
Police can actually arrest each player who make an advertisement regarding illegal activity.
Ad like this: "Join CB XXXX for criminal activites" is an illegal ad and the police can arrest the author as they have the signature and phonenumber of the person who made the ad. Pretty simple.