I will admit upfront that I have not watched many webcasts and tutorial videos, so my experience comes from a limited selection. However, the fact that I have not watched terribly many of these is not entirely by accident. I've watched a few, and by and large they weren't very good. There were a couple of reasons, but the biggest were that, rather than being instructional videos on concepts and ideas, they were walkthroughs and screencasts of people doing tutorial coding by hand.
I learned nothing from them.
Introductory books, on the other hand, tend to provide you with the generic form of an idea before providing examples. And one of the things I like about learning to program from books is that I can't copy and paste it if it's in the book, on print, in ink. That might seem strange, but I feel like I actually internalize and remember things more when there's a tactile element to the process, and even transferring code onto my machine from the book helps with that.
I don't doubt for a moment that other people learn differently, and can learn better from webcasts than from books, but in my opinion, the amount and nature of what can be learned from a large portion of webcasts, screencasts, and other videos is not structured well to promote the best kind of learning.