When the code is under a non-permissive license, that means by definition that GitHub cannot derive any usage rights from it. However, displaying the source cod

When the code is under a non-permissive license, that means by definition that GitHub cannot derive any usage rights from it. However, displaying the source code on the website (even to a logged-in user! even to the original rights holder!) is a type of "usage". Therefore they would be in breach of copyright. ToS is the only place where they can derive the necessary usage rights. In certain jurisdictions, usage rights may be conferred by an implied-in-fact contract. But for a company like GitHub that operates almost globally, it's probably a very bad idea to rely on local legal interpretations of the scope of implied-in-fact contracts.