It's a little presumptuous to assume that everyone who could possibly need access to a technical book has a loose $15 to spare. I know I didn't when I built it, as a 2nd year undergrad not living in a First World economy. The authors who chose to make them free obviously had a wider audience in mind than people who can afford their books. I can, and do, buy books now that I can afford them, but you'd be hard pressed to find a better introduction to Python than Zed Shaw's Learn Python the Hard Way, a free book.