GitHub's actions are not required per se, they just need to accept being civilly liable for any alleged infringement if they want to ignore the takedown notice.

GitHub's actions are not required per se, they just need to accept being civilly liable for any alleged infringement if they want to ignore the takedown notice. This requires a high-level decisionmaker, and there are a lot of takedown notices, so only relatively small sites can get away with making decisions on a case-by-case basis rather than doing blanket takedowns of infringing content. Like, there's nothing stopping you from responding to DMCA takedown requests with "this is clearly non-infringing and/or fair use, if you want to sue us send servicing to the following address". You just have to be a particular kind of ornery and make the judgment call that you're willing to risk being sued over protecting third-party speech.