This excuse for worker exploitation has always baffled me. 1. The contract was signed under duress: "Take this shitty job or continue to be unemployed." Consent made under duress is not consent. 2. It was agreed to under an extreme power asymmetry: If you refuse the job, your family will have to go hungry, but the company will easily find some other desperate soul to fill the position. 3. I haven't even begun to touch on all the class, racial, ethnic, and gender discrimination that workers face.
There's nothing even close to meaningful or fair negotiation under such conditions.