They're paying plenty of tax whenever they cash out.
For example: "After cashing in more than 9 million shares valued at $3.7 billion last year, 16 Google insiders will owe the Golden State as much as $380 million in taxes -- enough to cover the salaries of more than 3,000 state workers." http://www.boston.com/business/taxes/articles/2007/01/06/goo...