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.