Most of these polls just use a cookie to record (on your computer) that you have voted. A script can easily accept, then delete the cookie. This makes the poll page think you haven't voted yet. Repeat hundreds of times per minute, and you have electronic ballot-box stuffing.
A much more sophisticated approach would be for the web server to at least record the voter's IP address when they submit their selection. This would be much more difficult for most folks to work around.
It wouldn't work because alot of people are logging in via dynamic ip addresses through outfits like AOL and MSN. If someone ran a script through a dynamic connection and the server noted the ip address where the multiple votes were coming from and blocked it, the DU user just logs off and then back on and gets a new ip address, while the poor AOL sucker who next gets the old/bad ip address reassigned to him is now blocked from accessing that poll or maybe even the site. No business would want to block potential viewers on that basis.