Google seems to have removed the restriction to publicly share posts which were originally shared with a limited number of users.
The public option was totally disabled for the share feature.
Now, the author of the original post gets the ability to share the same post with public.
This comes handy when you accidentally share a post with some circles that was aimed for public sharing.
Another good use is to share a post with yourself while you are still working on it. And once it is complete, share it with the world!