Is there any service provider whose TOS compels them to provide service unconditionally to the general public? If you were running an SaaS service, wouldn't you want some recourse if a customer was abusing your service?

If you're a large customer and you're really concerned, you can probably negotiate a contract with GitHub that guarantees your access to the service. But if you're a large enough customer to warrant a custom contract, then no rational service provider would unexpectedly cut you off, because your business is valuable to them.