Regardless, not honoring a DMCA would mean GitHub is asserting that they think it's false and would need to defend their decision if RIAA were to sue them; it doesn't make sense to verify every single DMCA claim and increase liability by not honoring some.
THis is likely a consideration. From my decidely non-lawyer viewpoint the RIAA is using a terrible law, but still a law and they have grounds under this law for their claims. GH and MS have deep pockets and if they tried to initially fight this I'm confident they would settle. If you're the RIAA who would you rather go after - someone who will pay after a little bit of fighting, or a huge group who will immediately roll over but have nothing to give you?
