Microsoft have a public statement that they don't use proprietary code, only public code with public licenses. They have a lot of companies as customers who uses github, and they also use a lot third-party code in their own products.
Even BSD et. al. have attribution requirements - that must be a vanishingly small amount of code to be used. Me thinks the people who run GitHub (who have apparently decided to abandon the core business for the latest fun project) aren't being entirely upfront.
