The truth is that we actually do pay, as we have private repositories. We ask that people prepare their initial open source releases during a 30-day window - making sure they have a decent README file, the right LICENSE, Code of Conduct, etc.
Full disclosure, I work at Microsoft within our Open Source Programs Office and I own our billing relationship with GitHub for the large organizations including Microsoft, Azure, etc. We pay via a yearly invoice for a set level of services today but we're always looking for better ways to collaborate with GitHub!
For our externally-facing open source projects, we are using the GitHub.com public product like any other GitHub user - we realize that GitHub is where open source lives these days!
That said, we have some unique challenges, as we have nearly 10,000 members in some of our organizations.
Full disclosure, I work within the Microsoft Open Source Programs Office and am responsible for our GitHub.com billing arrangements at this time.