I'm really not sure how much value the stock options would be to the shop floor workers, they will be locked up well past when they are worthless, much as it happened in the Dot Com bust, but still his threat is a direct violation of the NLRA, which prohibits penalizing workers for exercising their labor organizing rights:
The United Auto Workers has been Elon Musk’s target for days of derision on Twitter, and his posts may open Tesla Inc. up to trouble with U.S. labor regulators.This, is, as the saying goes, black letter law, and, if the expected complaint is filed, I would expect the NLRB to take some sort of action.
The UAW, which is actively trying to organize Tesla’s California assembly plant, has fired back with tweets of its own. But the more consequential outcome from the spat on the social-networking service may come in the form of unfair labor practice allegations made to the National Labor Relations Board, according to Wilma Liebman, who led the agency during the early years of the Obama administration.
Musk posted earlier this week that nothing was stopping Tesla employees in Fremont, California, from voting to join a union. But, he wrote, “Why pay union dues & give up stock options for nothing?” Liebman read this as a warning that the company would take away workers’ stock options if they succeeded in organizing the factory.
Nothing stopping Tesla team at our car plant from voting union. Could do so tmrw if they wanted. But why pay union dues & give up stock options for nothing? Our safety record is 2X better than when plant was UAW & everybody already gets healthcare.— Elon Musk (@elonmusk) May 21, 2018
“If you threaten to take away benefits because people unionize, that’s an out-and-out violation of the labor law,” Liebman, who’s done legal work for the UAW in the past, said in an interview.
Ironically enough Trump's NLRB might be more likely to take action than the Obama's, because the Trump administration is less enamored with Silicon Valley than was the Obama administration, though I would not expect much beyond a stern warning.