What grounds would there be to fight? I don't think GitHub has any standing to fight the sanctions themselves. And if an entity is sanctioned all US persons and companies must stop providing them services. Hosting a git repo for them certainly counts as a service, doesn't it?

Perhaps there is some grey area as to what qualifies as a service being provided to that entity. Is my fork still a service being provided to them? Or is it a service being provided to me? But that is still something useful to the sanctioned entity, so am I now providing them a service? In which case GitHub is still a party to providing them a service.