The only way to really protect yourself is with a contract that stipulates the repo and any clones a contractor may have access to must be destroyed upon termination of the contract.
Github is a tool for collaboration. The point of a private repo is to manage who you are collaborating with. Once I don't want to collaborate with someone anymore, there is no reason for them to have a Github fork of my code anymore. As I said, it's cleaner.
I'm fully aware that they can save a local copy (did you read my 3rd paragraph above?), and of the importance of contracts.
It’s about trade-offs. If you must have “trade secrets” amongst your code, perhaps you can keep that code available only to whom you trust will follow your wishes with it, and have separate repos to share wherein you do not care as much about collaborators “stealing” the code.
As soon as you let someone into your codebase, almost anything can happen, right?
There are cases where it would make sense to sacrifice risks related to code-copying in order to have faster/better development.
Also, what’s stopping anyone working at ANY company from taking the company code and doing whatever they want?
Code is seldom everything.
There's "nothing" stopping someone at a job from printing every single one of their corporate emails and putting them into a binder to save forever, even after they've left the position. Nothing except professionalism, a sense of business ethics, the law, and the fact that it's a huge hassle.
This seems like a problem that's not a problem. If you are working in a private repo then there should be some formalized arrangement on who owns the code and who doesn't, and that would be the originator of the private repo. If, for some reason, that relationship is less clear, then as you say other contributors can just create their own clones of the repo elsewhere. If the obvious relationship is one of collaborative ownership, then a public repo is the obvious choice.
Except if that were as easy and obvious as forking, the OP wouldn't be complaining about it, heh.
...which is not a github fork, and not what they're talking about.