I think the reasoning is that you can host projects on GitHub without them being FLOSS, since you can just start a repository and add code without ever adding a

I think the reasoning is that you can host projects on GitHub without them being FLOSS, since you can just start a repository and add code without ever adding a license. If you do not add a license, it is just copyrighted code, and cannot be redistributed freely. Some repositories take this to the extreme. There are some closed source software vendors that use GitHub as an issue tracker and collection of Markdown files. If GitHub wants to permit such uses - it's their party.