NAL - IMO if you agree to it, especially in writing, then it is legal.

What are the ramifications if you don't do it?

I doubt your superiors are taking a cut in wages so why should you?

To me, if an employee should incur OT IMO they absolutely deserve that extra money. Hourly employees like that are usually the front line of the biz and to see them earn some extra cash is nice. It helps them.