I've struggled with this question for awhile now. One reason is because it takes me longer to do a Reiki session then most other practitioners, as alot of them don't treat the back of the body, which cuts way down on the amount of time they have to spend with a client. A massage practitioner books a one hour session, so it's possible that they are trying to work within that time frame because so many people are familiar with it. The other explanation is that many were taught or believe that the Reiki energy will go to the areas that need Reiki when they are treating the front of the body, so it is not neccessary to treat the back of the body under this premise.
I was trained by both of my teachers to turn over the receiver and treat the back of the body and I have facilitated many releases (when stuck energy is dissolved in the body by Reiki) in the back of the body. Many of my clients have back issues and request treatment there. So, in my opinion treating the back of the body is an integral part of any full Reiki session I do.
I have also had many of my clients report that the deepest rest they have during a session happens when they are on their tummys (turned over). So I will continue treating the back of the body because that is what I feel serves my clients the best.
We all have to do what we feel is right for us, so if one person feels things should be done one way, then that is the way they should do them. If another person feels they should be done another way then that is what is right for them and both ways are good ways. If we could all see that all ways are OK, wouldn't it be a more peaceful, beautiful world?