Everytime someone invokes an argument how taking something away by law prevents some form of crime, my answer is a simple fact: Criminals, by definition, don't follow the law. Now, in some cases, that isn't a problem: Guns are an obvious example, because they take resources and knowledge to manufacture, and are physical objects that can be tracked. Taking these away by law works. But encryption isn't a physical objec…
By that argument, why have laws against stealing (criminals will just take stuff), or grevious bodily harm (criminals will just hit people), or verbal abuse (criminals will just keep shouting at people). I don't buy this argument at all -- if we ban encryption, except for government sanctioned encryption, it will be the easiest thing in the world to detect if anyone tries sending it over the open internet.
Stealing is the actual bad behaviour, stealing itself is bad for society, anyone who steals should be punished.
Substitute "encryption" for "stealing" then you know why you're wrong.