What Zynga is doing is pretty repulsive, as I previously commented in a related thread (http://news.ycombinator.com/item?id=3219437), but it is not accurate to say that "[t]hey have to fire you for cause for you to lose your options."

The overwhelming majority of employees at startups sign documentation acknowledging that their employment is at will and can be terminated at any time by either party for any reason, with or without cause. In relatively rare cases involving founders or high-placed executives, the company will sign contracts stating that, though the employment is at-will (i.e., can be terminated at any time for any reason without liability), the employee will get accelerated vesting of one sort or another in the event of a termination "without cause" or a resignation for "good reason." "Cause" is usually defined as willful failure or refusal to perform duties that continues after notice and an opportunity to cure, misappropriation or misuse of company trade secrets, commission of a felony or other action involving moral turpitude, etc. and "good reason" is typically defined as material reduction in compensation or duties, relocation to a remote area, etc. If you have an employment agreement that provides for such acceleration, then you are clearly protected against the Zynga-style threats described in this piece. If you do not, then you generally are not on firm legal footing but still may have some fighting chances.

What are those? If you can argue that an otherwise permissible at-will firing becomes impermissible because it is animated by discriminatory animus (race, sex, age, etc.), and you belong to a protected class, you could argue that the threatened firing is illegal and would subject the company to damages (which, of course, would include the value of the unvested stock that would otherwise have vested had the company not acted illegally to terminate your employment).

If you can argue that the ground of termination violates public policy, this might be a separate basis for claiming that the firing is illegal, notwithstanding that the employment relationship is at-will.

If you can argue that the company has given you implied promises that your employment would be for a certain duration, this also might take it out of the at-will category and give you fighting chances.

If you can argue that you were induced by fraudulent misrepresentations, e.g., to leave an existing employment based on the promise of equity compensation, or if the at-will language in your agreement is defectively implemented, or if any other ground might exist by which you can legally claim you got cheated or had some promise made to you breached, all this too can take this out of the pure at-will category as well and give you a basis for leverage.

To sum up, "cause" is not usually needed by an employer to terminate employment and recapture unvested equity. But you also by no means automatically lose just because your employment is at will. This is a complex area. With a lot at stake, it pays to get good legal advice to see if you can find a good angle by which to protect yourself.

A good legal case depends on legal rules that support it and, even more important, on good facts that motivate judges, juries, and anyone else looking at the case to want to go in a certain direction. Here, Zynga is providing all the good facts an employee needs to motivate people to want to slap them upside the head. That by itself is not enough. But if you find even one legal hook that gives you a sound basis upon which to attack what they are doing, then you can stand and fight. It is not easy, but sometimes you have no choice.