The fact that it is unvested shares changes little. The employees where lead to believe that if they meant a certain milestones they would receive stock. Now the milestones have been changed. The employees must now also meet new criteria for productivity to receive stock. They signed up for one deal and are now being served another.
Except the possibility always existed that they could get fired and not get that stock. The only thing that has changed is that Zynga is now saying that they're willing to fire you just so you don't get that stock.
Deeply unethical, and terrible for employee relations, and not at all in the interests of the company, but probably legal.
Even in right to work states, there's all sorts of reasons a company can't fire you, it's just the burden is on you to prove them. Pedagogical example is that your boss can't fire you just because you wouldn't sleep with him. If they fire you claiming it was just for performance, but you have evidence of circumstances that makes it clear it was because you wouldn't sleep with him, then you've got a problem.
Firing someone to screw them out of contractual rights you both agreed to is problematic. And even if the company claims it was performance-based, which they will have to, the CEO's statements basically give the other side all the evidence they need to refute that.
