Legislation isn’t robust
That people occasionally get caught probably does discourage some of it, but there's obviously plenty to go around. At some point you have to accept that the law won't get rid of some of this stuff.
That makes absolutely no sense at all.
The law helps with many social problems, but it's rather frequent to hit cases where it's either outright innefective, or the cost of enforcement exceeds the cost of the problem itself.
However, you can't expect to see a virus on every single page you visit on the web, because it isn't the norm. The presence is severely reduced because it is illegal.
Seems to me making it impossible rather than simply illegal was a much stronger driver of virus-free web browsing.