A Free man or woman cannot sign away their Rights.
I do remember a case where an employer made industrial batteries. Women were having children with birth defects which was shown to be from working around the battery plant. The employer then transfered all the women to desk jobs. The women filed a sex discrimiantion lawsuit and won. So the employer made the women sign a liability waiver saying they understood the possibility of working in the battery plant leading to birth defects in children. The women signed them. Nonetheless, when the women begin having children with birth defects, they sued the employer for damages and, again, won in court.
So the employer was essentially screwed. He couldn't prevent the women from working there, couldn't prevent them from getting pregnant and couldn't prevent them from suing *him* when the children came out with birth defects even though he made the facts known to the women and had them sign a liability waiver.
I imagine he went out of business or moved out of the country .
A policy such as this one is a less eggregious 'signing away of rights' than certain types of non-compete agreements, etc. Indeed, what would be interesting would be to see what would happen in a situation where both types of items were in play. I think an employee might be able to make a case that while the company couldn't be forced to keep him on payroll, it could not simultaneously refuse to keep him on payroll and forbid him from working elsewhere. I don't know how that would play out.