My gut instinct was that was a jerk move, but after thinking about it I'm not so sure. Here's my answer from Quora... I'd love to know what others think.
(On a side note, the article cited on Quora mentions that employees were asked to give up some unvested options, but that their vested options are not affected.)
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This is a slight oversimplification, but a typical stock option grant of, say, .4% of a company is akin to the CEO telling you, "for every year that you work for us, I will give you .1% of the company." For early startup employees who typically take below market salaries, this kind of stock grant is one of the things that makes working at a startup more worthwhile (financially speaking).
It's exceedingly rare that a startup grows to a billion dollar valuation within a few years of founding, but when that happens, it makes sense for the company to re-evaluate whether it's worth giving someone an extra .1% for one more year of work. When a company is young and not worth much, giving someone $10k in paper value as a loyalty bonus is a no-brainer; when a company is worth billions, giving the same person a 50 million dollar loyalty bonus requires a lot more thought. It might still make sense if they're the COO or CFO or something like that, but does it make sense if they are a "Lead Software Engineer" or "Senior Product Manager"? How many "Lead Software Engineers" could you hire with 50 million dollars of stock? Put this way, I think it's reasonable that Zynga wants to let some early people go at their current compensation rates (and California has at-will employment, so you can quit or be let go without reason at any time.)
Note #1: I think this sucks for the early employees, and I certainly would not want to work for a company whose founders have a history of these kinds of practices, but that doesn't make the practices unethical.
Note #2: My answer would be very different if Zynga were trying to take back options that had already vested.
I am not a laywer, so my assumptions about how at-will employment works might not be 100% correct.Edit