It's irrelevant whether it's standard or not. Again, the terms in the code licence (including attribution) do not apply to Github, because that is not the licence under which they are using the code. You grant them a separate licence when you start using their service.
If someone who isn't the author has uploaded code which they do not have a right to copy, they are liable, not Github. This is also clear from the Github Terms: "If you're posting anything you did not create yourself or do not own the rights to, you agree that you are responsible for any Content you post"
It's almost as if these highly paid lawyers know what they're doing.
