How do Buffer and HootSuite post to a user's Google Plus page/profile?

• The Google+ API is read-only
• The Domains API is for Google for work only.
• There's nothing else.

And somehow social media scheduling services like buffer or hootsuite allow users to sign in with their "normal" non-business google accounts and post to their G+ profiles and pages.

I can authorize with the user's google account, even ask for the following permissions which are needed to post, but there doesn't seem to be an API that makes use of this.

• https://www.googleapis.com/auth/plus.stream.write",
• "https://www.googleapis.com/auth/plus.login",
  • "https://www.googleapis.com/auth/plus.stream.read",
  • "https://www.googleapis.com/auth/plus.pages.manage",
  • "https://www.googleapis.com/auth/plus.media.readwrite"

How do they do this, seemingly without an official API?

There is an official API. It is however private and only available companies Google decides to partner with. For example here is a news report of Buffer getting access to the private Pages API.

He explains that the site was given access to Google’s API about two months ago, and “hopes to be first in line” if and when Google opens its API for personal pages

• https://techcrunch.com/2013/08/28/buffer-adds-google-plus-sharing/