Well, it's a joke because the problem becomes apparent after you think a bit about it. The exact same reasonig can be applied to anything illegal, criminals are criminals because they don't respect the law, so you could try to say that laws are useless. Reality is, if something is illegal not only someone can be punished after the fact, but in some cases also preventive measures can be taken.

Regarding the comment, it isn't going to stop anyone. Most people will not do cybercrime because they're honest. Of the remaining, the risk of being sentenced to jail time will instead stop some people, even if not all of them.