It is perfectly reasonable to have shares that vest conditionally based on future performance. Obviously this needs to be clear up front though (and the definition of "future performance" also needs to be clear).

I've always thought Zynga was a huge waste of space/energy/life/etc, and I hope no engineer with any other job opp ever joins again, but it goes against the whole definition and point of vesting to say that unvested shares are already yours. If you quit or are fired, you rightfully only get the ones that have vested, that's the whole reason they gradually vest - so that neither party is completely bilked if you leave after a year and a half. They're not handing over that percentage of the company on the day you join. That would be silly.

It's disturbing that Zynga is doing this, and it's a shitty thing to reneg on salary promises, but that's basically what vesting options are - uncertain salary promises. What they're doing is saying "We promised you a salary based on company worth, but we're now growing so fast in value that there's no way you're worth this much to us, so take a pay cut from now on, or we're firing you". If those people have been there for three years, they should already have 3/4 of their stock vested, assuming they're following the typical valley vesting schedule.

I think the main reason people are so vocal about this is because there's generally an implicit understanding that options are different from salary, more set in stone. Comparatively few people join companies based on promises to be paid 2*x salary the following year. This is upsetting a big part of the natural order of things.

Hopefully this won't impact more upstanding companies.

I think it is mistaken to view options as equivalent of salary. When you are granted those options, you have no clue what they will be worth on the day they vest, and neither does the company. This is why the options have to be valued at their worth on the day of the grant.

Options are used as an incentive for employees to stick around with the company and to work harder in the hopes that their hard work will result in 2-4 years in a higher stock price (or high IPO), which benefits both the employee and the company.

Attempting to take back options which were promised in the past because the company currently thinks the employee doesn't deserve them is aptly named "claw back", because that's exactly what it is.

Imagine a company asking someone in 2011 to give back a part of their 2009 salary, or be fired.

I agree that the connotations are different, but the way the vesting is written into these contracts, it is essentially a periodic stock grant, which ends as soon as employment does, and with no guarantee that it will continue. In that way it's very much like a salary, despite how it's typically viewed. Perhaps the expectation part of things is strong enough that this could be considered bad faith, but my understanding has always been that if someone doesn't work out as an employee for any reason, they have no claim to the unvested stock.