Poison pill shares are new shares, issued by the company and then sold to existing shareholders (with the exception of anybody who owns more than say, 15%). Since these shares are sold at a discount to the current market price, it is arguable that existing shareholders are benefiting as they now get more shares at a cheaper price, while also screwing over anybody who is trying to get >15%.

Now, you may argue that it is in the best interest of the shareholders to allow the hostile takeover to go through, but it appears that the strict mechanics of the poison pill do not immediately hurt shareholders.