Read the second link below (https://plus.google.com/+JulianBond23/posts/4mLaNh31JcW) which has a very deep and thoughtful comment from Thor about why Google+ does not have an API for posting content. I had thought of a few of those reasons myself, but there are a lot more than I realized to say no to that feature.
Originally shared by Abraham Williams
Thor Mitchell has two amazing comments about the difficulties Google+ Developers face managing the issue tracker and opening the API to programmatically posting. (Look for the really long comments)
Issue tracker: https://goo.gl/OLaFpf
Write API: https://goo.gl/13ozxb
The issue tracker comment is good too.