Virtually all of them do. An example of why they do this is craigslist, which did not disallow this and an early employee sold their stake to ebay, which caused

Virtually all of them do. An example of why they do this is craigslist, which did not disallow this and an early employee sold their stake to ebay, which caused craigslist much consternation. Though I'm not sure what's stopping you from just making up a contract that says I agree to sell you X shares for price $Y in the event that they become publicly transferrable and then selling that contract. I think options agreements usually contain language that say you're not allowed to do that, so maybe the risk is if you were to get caught doing that they could cancel all your options.