Anytime you have both vesting schedules and at-will employment, your employer can fire you at any time and you will not get any unvested stock. Startups need to do this all the time when employees are underperforming or a bad fit. Renegotiating so that you can get a portion of that stock and stay employed is perhaps, if this were an isolated incident, a much better deal for the employee than getting fired. Where this…
If employers are worried about a serious disconnect between performance over time and equity payout, they might cap the valuation of an equity stake by taking back call options with high strike valuations. They could then give more shares or extinguish calls in response to performance. But this would reduce the apparent value of the up-front stock grant, and thus the employee's incentive to come on board.
I don't know enough to support firm conclusions, but this doesn't sound like behavior I'd hope to emulate.