I understand that from your perspective as a CEO/Founder there are nuances here that make the business ethics not so cut-and-dried. But for the vast majority of talented devs, the take-away from all this will be a simple story:
Once upon time the "Google Chef" could become a millionaire.
Now he can't.
The End.
Again, not saying I agree with or would ever do what Zynga has done, but how can you not look at these two scenarios the same way?
Fortunately for you, dear founder, there's an alternative (at least there was, until these guys ruined it for everyone). Instead of requiring the employee to commit up front to stay for four years, you structure the compensation package in such a way that the employee has an incentive to stay, but isn't required to do so. These packages are often called "golden handcuffs" because they bind an employee to an employer for a number of years, the result you're after, in a way that relies on a large signing bonus of "maybe-someday-we'll-all-be-super-rich-won't-it-be-grand dollars" that is to be disbursed annually over the life of the agreement.
You and the employee both hope those dollars will eventually be worth something, but right now all you know for sure is that you have a lot of them. It's a trade that works for both parties; you keep your employee if those dollars prove valuable enough to compensate for the opportunity cost incurred by the employee in continuing to work for your company, and the employee has the freedom to move on if they don't. It's the "golden" part that keeps the employee around, not the "handcuffs". And remember that it was you, the employer, that chose to do the deal this way. You would have preferred regular handcuffs, but didn't have enough "we're-a-startup-and-we-don't-have-many-of-these dollars" to afford them.
But, ethics and nuance aside, I do think the implications in the long run, esp for founders and entrepreneurs who actually need the 'Google Chef' story, will be a simplistic account in the minds of developers that will make it harder to recruit them.