Some of the speakers I've met at conferences were people on who's contribution in form of articles/tutorials/videos I relied on (and still do) to learn and develop my craft. I was so glad to be able to thank them in person. They all turned out to be great people in real life as well. What I'm trying to say is that, sometimes, some people do it for free just because they care about others and love what they do.

edit: spelling, missed a ".".