http://www.sec.gov/Archives/edgar/data/1011006/000089161808000305/f41347a4defa14a.htm

Not only will the poison pill be introduced when Microsoft buys Yahoo, but also when a different board is voted for by the stock holders (eg Icahn) (change of control)

Is this legal pratice? You are basicly taking your stockholders hostage to vote you on the board.