I would be OK with such a law if it only applies going forward, to companies that are headquartered in SF and formed only on this date or later.

But to pass an IPO tax after hard-working employees have put in literally years of longer/more-stressful hours at reduced compensation levels (relative to working at established companies) to get to this payoff is unfair and unjust. Those employees have entered this arena (the startup game) with certain expectations and rules in mind, which made taking on risk and deferring gratification worthy to them. Pulling the rug out last minute to shore up the city's own finances is punitive and unreasonable, especially when the law is targeted so narrowly. And no, the fact that some of those employees will get large sums of money out of this does not make it less unfair or less unjust.