The teacher isn't obliged to work for free - I didn't write that. I wrote that a teacher whose worst problem is a too high fraction of overachievers trying to get into Princeton among his or her pupils has no reason to complain.

An actual overachiever would do everything possible to get an A on the first try - get feedback on assignments and projects before submitting them, ask lots of questions to understand the material, pay attention in class, come to office hours, study harder etc. I think most teachers would welcome that variety of overachiever.

This is just wasting the teacher's time with whining and grumbling.

Er, you are clearly failing to take into account unforeseen circumstances. Perhaps the pupils knowledge actually warranted an A grade but he or she had a cold and weren't able to perform at top level during the exam? Maybe he or she misunderstood some task description? Etc.