They can't just take away equity that's vested to you. And any equity that hasn't vested isn't yours.
http://stockoptioncounsel.com/blog/standards-ownership-canth...
I never saw anything like this when I worked at an early stage startup, and it's obviously something an employee who reads their employment agreement would ask to be taken out. I'd never work at a company with that clause.
On the other hand, this can lead to disaffected employees who are only staying with the job because they are waiting for their options to mature.