How to share a Google+ public post outside of the Google+ app?
Tap the post you want to share
Open the overflow menu (three vertical dots)
Tap "Link"

You'll receive a message: "Post link copied to clipboard."

You can now paste that link to whatever sharing mechanism you'd like


Not the same as using the Android Share function, but, again, Google+ is not alone in doing this. I don't know when this function was added to the G+ app.

Of course, it only works for Public posts.