Because this isn't something you posted on Facebook, this is just visiting a site and as a consequence having an application added. As the article points out, though the app doesn't show up on your profile, a friend who looks at the application home page will you see you in a list of "friends who have this application."

I agree it's hypocritical to post something on a Facebook wall and then complain if someone reads it; you post something, it's public to some extent, and you'd better be ready to face consequences of that. But this is an admitted bug that allows friends to see if you've visited a site without you having taken action other than visiting the site. That's just plain wrong.