To be fair, people are asked to take cash pay cuts all the time. They're talking about unvested stock (future compensation). It's tough to defend this, though. I would assume that it would be a breach of contract, but I'm no lawyer.
Here it seems people are being asked to take a pay cut not because their company is having problems buts because is doing very well indeed.
But... he could just fire them. Which is a problem in itself.
This is why I have a major problem with typical employee stock options. It is just flat out not worth as much to employees as it is to investors and founders. Employees are basically put in the position of having to trust that their management (and their management's acquisition overlords) will do what they said they'd do. Between the employers and the tax man, employee equity can be a real bitch.
Translated into english, most stock option agreements say, "We'll give you XXX options per month (after you pay us for them, of course). IF we feel like it - we can always fire you and owe nothing more, as you know. Taxes are your problem (good luck!). Oh yeah, and you don't get shit for a year."
Ever try to improve the situation up front? Oh, the howling and moaning you'll witness from the company and their lawyers. ("That's highly irregular! Nobody does that! It's a standard vesting period! Why are you so greedy! You could just walk away with stock!")
So Pincus should be willing to essentially write futures contracts on the proceeds from his own shares and trade those with employees, who should be willing to accept them in trade, even at precisely equal expected value.
Meanwhile, he's likely to see an exodus over the coming months of other good employees whom he didn't try to screw (this time) but who also now realise that the benefits of staying long term aren't necessarily worth the paper they're printed on.
You can pretty much write the rest of the story when something like this happens. The only question is whether people like Pincus himself will manage to cash out at some stage before Facebook intentionally or otherwise pulls the rug out from under Zygna in much the same way and the potential value of an IPO plummets.