I guess these agreements mean that the property isn't full unrestricted property of the employee... and therefore income tax isn't payable when they vest.
The tax isn't avoided - it would just be paid when you sell the shares instead - which for most people would be a worse deal because you'll probably sell them at a higher price than the vest price.
It's a worse deal in retrospect for a successfull company. But there and then it's not very attractive to pay an up-front tax on something that you can sell at an unknown price in the relatively far future.