Obviously, if you're leaving Uber you'd be wise to buy your vested equity if you can.
Obviously, if you're leaving a company that is crumbling, you probably shouldn't.
But what about an earlier, Series A/B startup that's promising but still shows a lot of risk? Or what about a later stage startup that is doing well but not on a guaranteed path to a huge exit? On what information would you decide whether or not to exercise?
That's their current policy.
If you were an early employee, you may have had the option to exercise your options early (while being an employee. Only being able to exercise your options when you leave is a myth -- one that I used to believe myself).