I've never understood why people say things like that. Just like bike locks keep honest people from stealing your bike, society in general keeps nutcases from going on shooting sprees.
I'd guess that any reasonable marksman could pull something like this off. THANKFULLY most people have the decency not to go around shooting at random bystanders. But if they didn't, well, there'd be a lot less bystanders. It's not the police/whatever that protects us, it's human nature (including cowardace and fear of retaliation if caught) that keeps us from having people go berzerk and shooting their neighbors on a weekly basis.