Can someone explain to me how you can fire someone that already declared to leave and do financial damage to the employee with that?

I guess it's some clause of a vesting schedule that gets triggered when being fired and he already declared his resignation before his shares were vested, but they would've been after his last day?

I find it so weird that he already quit the job but the company could actually save money by firing him. Shouldn't something except fear of a bad reputation prevent this kind of move to do actual damage?