You see, unlike free hugs, where the person offering the so called "free" hugs gets an instant reward by being hugged for free, the free back scratching does not guarantee anything in return.
Most people will be happy to get a free back scratch and move on.
Some may return the favour, but most will not.
Some may even attempt the most inefficient method of dealing with the issue, using backstabbing as payback. Inefficient, because if done properly, and most people have mastered the art, it can happen only once and it does not address the back itching problem very well.
On a relatively unrelated matter, the registrations for the second International Outsiders Festival are open:
www.outsidersfestival.blogspot.com