Short answer: because drug dealing gangs are organized like McDonalds franchises where most "foot soldiers" make minimum wage or less while the leaders make hundreds of thousands of dollars.
Side note: I wish TED talks were available in text format. They're always interesting, but most of the time I'm unable (or unwilling) to hunker down and watch a 20 minute video. Web content needs to be scan-able.
For those not acquainted with the book: "Freakonomics"<p><a href="http://en.wikipedia.org/wiki/Freakonomics" rel="nofollow">http://en.wikipedia.org/wiki/Freakonomics</a>