Pre-ipo transfer restrictions are pretty normal. It never even occurred to me to try selling Google pre-ipo. It wasn't really a thing before secondmarket came along, and arguably trading on those markets is taking a company public without their consent. If I ever start another company, I would definitely have transfer restrictions. It's pretty important to control your equity, and I think the day-trading mentality is…
Equity is compensation. You can't tell your employees what to spend paychecks on, and you shouldn't be trying to block them from gaining liquidity. Maybe a doctor just told them they have a year to live, and won't be around for your planned 2019 IPO.
Unless you are granting them voting shares it is unfair to lock them into your vision of the company going forward, especially if they have departed.