Quitting after you've vested is 100% honorable. It's unethical to fire someone the day before they vest because that makes it clear you did not intend to ever

Quitting after you've vested is 100% honorable. It's unethical to fire someone the day before they vest because that makes it clear you did not intend to ever hold up your end of vesting agreement; it means that you entered the agreement in bad faith and planned to manipulate your way out of paying up. But in the case of quitting after your options vest, the employee never made any agreement - explicit or implicit - that they would continue working after options had vested. If the agreement is that the employee will receive 1,000 options for 365 days of work, the employee is not obligated to work for 366 days any more than the company is obligated to release 1,001 shares. Somebody who works 365 days has already held up their end of the bargain. They didn't cheat.