The case is Circuit City Stores, Inc. v. Adams but it doesn't really turn on constitutional issues like that from what I can tell. I suppose the reasoning is you're both equal parties and you can just turn down the employment if you don't like the provision (this argument is a ridiculous fiction since you need to work to live but whatever).

I mean honestly I think this is ideologically motivated just like the rash of decisions that have practically made it impossible to file a class action lawsuit by letting companies require their customers to use arbitration.