I'm not defending the move, but in reality the company could terminate the employees (at-will employment in California), which would terminate their further vesting anyways.
This is not the clawback scenario that Silver Lake executed with Skype.
I also can't see how employees who give up their stock and remain with the company can possibly have a healthy relationship moving forward.
Sounds like Pincus gave performance-based pay with a pretty transparent system, which should have worked well. Employees must know how management sees their performance, otherwise you're destined to have a bad breakup.
If the story is accurate (hopefully it's not as who'd want to ever work at Zynga after this?) then the lack of stock for new hires was a management issue, and should be resolved from management, the investors and the board's pockets. After all, each investor saw the cap table before putting money in.