Not surprising you would not understand this, given your lack of experience.
The most likely response to this proposal would be a combination of the following: One, in most cases it will create an incentive for employers to pay a lower salary, because of the prospect of having to pay overtime. Second, it will encourage employers to avoid overtime by hiring a second manager, or two part time managers, to avoid the overtime. It will make it more difficult for the bright, energetic manager to work long hours, get promotions, move up the ladder and make more money. It creates and legislates mediocrity.
Just like Obamacare has created a whole new category of part time employees know and the "29ers". Employers have created a new category of employee to avoid the costs of government mandated health care for employees. This has acted as a ceiling, and is keeping people from full employment and higher earnings. It is what happens when you level playing field types get their way.