I got the same thing because I was signed into gmail. After I logged out I could read the post w/out having to accept g+.

No you don't.

You need to join g+ only to reply to the post, but didn't I need to join HN to reply to your post?

Were you logged into a google account? I was logged in and got the above screen. I was not trying to reply--kind of hard to when you can't even read the post.

If I open the link in an incognito window, I can read the post just fine.

I didn't get a prompt on desktop or mobile.