Digg does not show the full article but only a teaser. If the visitor is interested in the full article, he will click on the link so it's a win-win situation.
First of all, there have been several occasions where a digg user copies the entire article over to Digg in a comment. Second of all, say I write an article about red widgets, called "10 reasons widgets are red." If a searcher does a search for that phrase and Digg comes up before me, then Digg will likely get that visitor. Whether the person who submitted the story did a good enough job of selling it is what determines whether or not Digg will determine if they click through to my story. That's like asking the searcher "Are you SURE you want to go to this site?" I don't know about you, but I'll take every visitor I can get, whether they like my articles or not. If they don't like the article there's always a chance they'll click the ads to leave and even if they don't, I've increased my impressions which is of some value. So, no matter what, I want those visitors. It is not a win-win situation.