> is not that the worker is being made to work for free but rather that the worker has not agreed to be paid by any hourly measure but rather for an overall performance to be rendered, no matter how many hours it takes.
While it's nice to think of being paid for the completing tasks rather than hours worked, it is less common for people to work fewer than 40 (or some standard number) hours than more. In fact, if an employee was completing his tasks and working only 25 hours a week, many employers will increase the workload, since they aren't making efficient use of the employee.
The reality is that employers like to frame overtime in terms of paying for services rendered but when things swing the other way, employers like to think of things in terms of maximizing utility.