If you are salaried and can complete your tasks (job description) in under the alloted time, is the onus on you or your manager to find more tasks? Why is ok f

If you are salaried and can complete your tasks (job description) in under the alloted time, is the onus on you or your manager to find more tasks? Why is ok for your employer to make you complete additional tasks, draining you of your excess energy, but not increasing your salary at all? When there exists this productivity/energy gap, the default US view seems to be you should give it up to the employer, but the article author is instead keeping it for himself.