A lot of the genetically modified organism (GMO) push comes from other private interests. Agriculture isn't the easiest business, given profit margins, weather, and other factors. Companies want to differentiate their product however they can, and that can mean vilifying their competition.
It's similar to saying “Honey Nut Toasted Grain Circles, 100% uranium free!” It's true, but meaningless. Statements like this can give the impression a product is somehow different from, or better than, the others. This actually happens: “Antibiotic-free eggs/chicken!” All poultry farms in the USA are banned from selling product with antibiotic residue, so it's a moot statement.
“Antibiotic-free milk!” - same thing.
If any farmer puts antibiotic-laced milk into the tank when the milk is picked up, the entire truckload of milk has to be dumped. The farmer then has to pay market price for the lost milk to all the other farmers who have contributed to the tank. They are very, very careful not to do this. Cage-free is a misnomer, too; they rarely use cages anymore.