Much of the world needs to work two jobs. Chris Williams writes that managers should be careful in how they react to an employee working multiple jobs.
My point is more that salaried employees, by definition, are not required to put in a certain amount of hours. That would make them hourly employees. All salaried employees are required to do is to complete their work by a deadline. What that work is and what the deadline is are usually not defined specifically in their contract, because as you said, both those things constantly change, so it would be impossible to reflect that in some binding agreement.
My point is more that salaried employees, by definition, are not required to put in a certain amount of hours. That would make them hourly employees. All salaried employees are required to do is to complete their work by a deadline. What that work is and what the deadline is are usually not defined specifically in their contract, because as you said, both those things constantly change, so it would be impossible to reflect that in some binding agreement.