The problem with rambit use of antibiotics is that the 'bugs' become resistant to the antibiotics so when we humans need them they no longer work. Its not necessarily a big issue for the meat we consume. Its just do we as a society want any of our existing drugs to continue to work?
For a very short period of time I sold a product called Fish Mox which is simply Amoxicillin. There is NO WAY that the quantity I was able to sell these in where being used for aquarium fish. There aren't enough aquariums in the world to cover how much of this was being sold. Simply put Fish Mox was a very inexpensive way to get an antibiotic without a prescription.