Digg, like many other sites, have a page where users can enter a series of e-mail addresses to invite people to join the site. What exactly is the difference between that and spam 1000's of e-mail addresses with an invite to the site? Isn't it just indirect spamming? And is that legal?
I'd guess it has to do with the fact that they'd assume you're referring people you know, and since it's not coming directly from them, but rather through you, it's not exactly "commercial."