The bottom line here is "You don't get to be a part of the IPO windfall." Given that the employees that are getting stung like this have accepted the risk of working at a startup, and likely have accepted lower salaries in exchange for the promise of stock options, this is morally equivalent to theft of services.
Stock as a compensation structure only works when people use it in good faith. Scummy moves like this are only going to make it harder for startups to attract talent unless they can pay full market rate right out of the gate.