It’s if SpaceX decides that the employee misbehaved, not the company that misbehaved, that allows them to ban that employee from selling private stocks. Also if you leave the company for any reason you lose out on 6 months of them or if you are fired they will buy back all of your stocks at $0, all the while you are paying taxes on your stocks. So the employees could possibly end up losing money from taxes on something they never actually got because the company said so.
Also, i guess the next question is what counts as misbehaved since the company gets to decide? Does calling in sick for a day count? What about if they want you to do something that they legally can’t ask you to do and you say no? It sounds more like a ploy to control the workers even more, whether or not what SpaceX does is legal. Sure if its not legal they can sue and maybe even win, but SpaceX could drag out the suit till it has to be dropped from a lack of funds on the employees side or make it not worth the amount they would win.
