From Wired:
First of all, documentaries are incredibly important records of our history and culture. They’re visual histories, and they’re increasingly based on copyrighted culture. Our book describes several instances in which the telling of that history has been thwarted by permissions issues. An example is Jon Else having to pay $10,000 for a four-and-a-half-second clip of The Simpsons playing in the background of his film (Sing Faster: The Stagehands’ Ring Cycle). The makers of Mad Hot Ballroom had to pay that same amount to EMI because a cell phone rings in the background of one of the scenes, and the ringtone is the theme from Rocky. These examples really resonate with people. They understand that these are instances where copyright is not working the way it’s supposed to.
Here.