The online material is a virtual lab where you can test out the things you learn in the book by breaking actual web applications. Technical books on niche subjects rarely leave the authors rolling in money, so it would be pretty silly to expect the labs to be free.
If you are too cheap to spend $7 an hour, you can set up a vulnerable VM and accomplish the same thing for free.
If you are too cheap to spend $7 an hour, you can set up a vulnerable VM and accomplish the same thing for free.