So, while Elon Musk is an asshole, and these terms are awful, there is more to it:
They involve stock awards, which are given by the company to the employee. They can either be options (which give you a choice to buy at a certain price, and presumably you wouldn't do that unless you can sell for higher price), or outright stock grants . They are given as part of compensation, and vest on a set timetable. (So, if someone was given 1000 shares, the employee would still see 1000 shares in some account, but they may only be able to access 100 of them every six months). So, this whole discussion is about shares the company gave to the employees in the first place.
Then, the other wrinkle is that SpaceX is a private company. That means that employees can't just go sell their shares on the open market. So SpaceX graciously offers to buy back these private shares at whatever they think they are worth at the time. While this sounds fishy, the only other real alternative is for the employees to hold on to the shares and sell them if they go public....
.... However, simply receiving the shares when they vest is a taxable event. So if SpaceX didn't offer some way for mere mortals to turn their shares to cash, then in effect they would be saddling them with an enormous tax burden and no way to raise the cash to pay it. So they have to do it this way.
Do they really need to confiscate an employees shares if they hurt Elon's fee-fees? Of course not. But that's the only dumb bit here. The rest is pretty standard for a private company who attracts workers with stock benefits.