Microsoft/Github makes a reasonable attempt to remove infringing code. If you obtain a copy of proprietary source code owned by Apple on the dark net, and upload to github, they'll definitely remove that. If companies were responsible for user-uploaded content that the company takes reasonable steps to remove, no one would be able to accept user-uploaded content in the first place.

Facebook wouldn't be able to allow users to upload photos.

Hacker news wouldn't allow me to post this comment (someone else could own the copyright, right?)

If Github's ToS says that they have carte blanc to do whatever they want with FLOSS licensed code, including to relicense it, then either every single codebase on github violates their ToS or their ToS violates every single license and therefore the law. A reasonable attempt under these circumstances would be to remove every single FLOSS licensed repository on github, so I'd argue no, they do not make a reasonable attempt to remove infringing code.

You are conflating many different things.