But... the whole arbitration thing is itself a reaction (in many cases^) to general suckness of the legal system which is primitive, clunky, slow and often more trouble then the trouble it's trying to solve. It's wide open to abuse, and a lot of what we call law is in practice a mature "game" based on abuse of the legal system.
So, yep.. these should be ilegal. But we also need a legal system capable of functional resolution of these disputes fast, without abuses, mutual destruction stalemates and burdensome "costs" including eventualities such as "as an employer, you will always be in labour court no matter what."
That said, sexual harassment in the workplace is an "our vision of society" type of problem. There's a case for giving it it's own courts or public dispute resolution frameworks.
Law really needs to evolve. We are not settling disputes between norman kings and saxon lords any more.
^sometimes it's just naked self interest. Ie, We will win more often in private arbitration, so let's write it into the contract and hope they don't notice or are not in a position to object .