Why do instructions for carving a chicken disregard the back?
Back meat on poultry tends to be rubbery, inaccessible, and there is relatively little of it. The meat is almost like other dark meat but is found only in thin sheets. Also since during traditional roasting the back meat is down in the pan it tends to be less cooked than is pleasant for dark meat.

It isn't practical to try and carve it because it is a ton of effort for just a little bit of not-very-good meat. 

After carving off the major cuts I will use my hands to pull off any useful scraps- including some of the larger pieces of back meat. I save these pieces for my standard poultry scraps applications: pie, soup, and enchiladas.

Whatever is left gives up it's goodness into the broth when the carcass is boiled and is then fed, with the spent carcass, back to the chickens.