Giving the content away for free publicizes the event and adds to the cache of (and willingness to pay for) the actual event. Also,

Anderson did not stop there. He opened up not only the TED talks themselves but the TED name. TEDx are events that can be put on by pretty much anyone. You need a license and have to do a good job (there’s no automatic renewal of the license), but nearly anyone can pitch in. This is literally a freeing up of the concept “ideas worth spreading” to allow anyone to select what those ideas are. So long as you follow a few simple rules — a talk format, some video, and no ads or other commercial tags — you can host a TEDx event. And there are now hundreds of these each year. What is more, TED regularly features talks from these on the site, so they act as feeder for TED publishing.

That’s from Josh Gans, more here.