If Microsoft's CEO places 20% of his stock up for grabs, wouldn't that arouse a heck of a lot of suspicion? And, if he puts 95% up for sale, it'd arouse even more suspicion. Not sure what the current laws allow, but let's say it's 10% max, before the SEC must be notified of the sale (it's probably a certain number of shares though).
I'm not sure the current rules would keep the CEO from getting richer - 10% of the stock for current price, vs. 99% of the stock for 10% of current price is equivalent. BUT the latter lets everyone know there's a problem.