Why? Do you prefer for HN to be a place where only TechCrunch, NYT and Mashable get submitted? Such a button would greatly improve the variety of submissions here.
Any user that recognizes de HN button knows how to submit to HN anyway. Too much visibility of the site has lead to connectivity problems. And surely it's no good to HN become a clone of reddit, digg, delicious or whatever. HN is better being a different taste of internet.