These medicines treat common illnesses and infections that people who own large animals and livestock are very familiar with and qualified to treat. They have been trusted to do so for as long as these medicines have been available. The first chicken antibiotics were introduced in 1948. Since then, there has been no call nor reason to regulate these items. Many farmers and ranchers tend to many of their animals' veterinary needs, which is a part of being a responsible animal handler who needs to keep costs down.
It would be ridiculous to take a chicken to the vet. When chickens are sick, you consult with other chicken owners and go buy the needed items to make them well again. It was a good system.