You have to sign in because you're signed in to a Google account but not Google+. If you were either signed in to Google+, or not signed into Google at all, you would see the article without a sign-in prompt.
That's just plain stupid and annoying. I didn't notice yet since I've abandoned my Google Account a few months back and am therefore not signed into Google at all.
However, why do they do that? Hasn't it occurred to them that not everybody with a Gmail Account wants to be on G+?
