The reason behind the "rule": that you can only call it an organic egg if you feed your chickens certified organic feed AND pasture your chickens on certified organic pasture::
Non-organic feed has been exposed to the pesticides and whatever other chemicals that were used to grow those plants. Science shows that the plants take up those chemicals and store them in their tissues. Then, when the chickens eat those plants/seeds/grains, they consume those chemicals also, which become concentrated in the chicken's tissues.
The chemicals then get into the egg as it is being formed. Chemicals designed to kill insects are also toxic to other living creatures.
My husband and I have discovered that we are much healthier now that we eat all organic. We save money on hospital bills from chronic asthma, and we're happy to spend it on organic food instead.