Comp clawbacks in exit agreements, that weren't part of the employment agreement?

I've seen equity clawbacks in employment agreements. Specifically, some of the contracts I've signed have said that if I'm fired for cause (and were a bit more specific, like financial fraud or something) then I'd lose my vested equity. That isn't uncommon, but its not typically used to silence people and is part of the agreement they review and approve of before becoming an employee. It's not a surprise that they learn about as they try to leave.