Foxnews runs the video over and over again, without having bothered to check whether it was for real, because it fit the politics they're pushing. When that happens it's called propaganda, not news.
Fox News Caught in a Smear: Media Matters calls for accountability following shameful omission during ACORN reports
And it didn't take much digging to begin uncovering the truth about the Rightwing political operative that Foxnews calls "a filmmaker":
Right Wing Darling James O’Keefe: The Man Who Exposed ACORN and Lucky Charms
Zachary Roth at TPM Muckraker gets to the heart of what's really going on here:
Conservatives and Republicans have longed waged a campaign against ACORN, focused mainly on the group's voter registration activities. During the 2008 cycle, ACORN, by its own tally, helped register over 1.3 million low-income, minority, and young -- in other words, predominantly Democratic -- voters across the country.