I would blame this one on the USDA. They are the ones that came up with this term.
Further little tid-bits is that the "farmer" just has to open the cage door for an hour. The chicken never has to leave and it is called cage free... or some such. You can look up all the definitions. The are nothing close to what anybody (who hasn't seen factory farming) would envision by these words.
Its like whey in ice cream. Why would they use whey? So they can call it ice cream.