No one has to sell. That part is optional. This allows employees and former employees to receive shares and to pay the taxes due on receipt. Paying taxes at low valuation is a win.
Obviously no one is literally forced to sell. Is this really that hard to understand? This entire thread is about how by not going IPO, the employees were unable to sell. I'm sure they would have preferred to sell a few years ago instead of paying less taxes now.
It also seems that some employees' options are expiring, so while they aren't literally forced to sell, in reality, they are.