If people can opt out, then this gives the opportunity for the business owner to make opting out a requirement to get the job. Then you may as well not have the protection at all. You can't opt out of:
- minimum wages
- OSHA
- discrimination protection
- etc.
There's no special reason why you should be able to opt out of this work hour regulation. Now if the regulation is overall a bad idea, that's a separate question.