Why do we need an opt-in spam law?

As you saw in my previous blog post, I've come out in support of opt-in being the legally mandated permission standard in Canada. I don't think it's all that big of a deal; as I said before, opt-in is already a best practice. In response to that, one of my Twitter followers asked, what's the point? "Why, if opt-in is best practice in Canada does the government have to get involved?"