If someone says, "You're salaried. We're giving you a task to accomplish, rather than hours you need to have your butt in the chair. Sometimes you may need to work more than 40 hours in a week," then isn't it reasonable to say that if you get your job done in fewer hours some weeks, there's no obligation to cast about looking for ways to keep busy?

If a job regularly has too many hours for the work, the company should then look for more work, and if the job has too much work for the hours, the company should look for more employees.

Personally, I'm neither an employee nor an employer. I've been both, and prefer to be my own boss and the boss of none.