I have looked around and found this page to best explain the bad about packaged dog foods. It also includes what is considered byproducts. The nutritionally rich heart, liver, and even gizzards of chickens are sold with whole chickens for human consumption because the giblets are used for soup stocks, gravies etc. So what's left from chickens for pet feed are the heads, beaks, bowels (with fecal matter in it), feet, bones, and even feathers.

Here is some interesting reading:

http://www.sojos.com/truthabout.cfm