Have you bothered to look into it? It's not like the information is impossible to find, even if they don't exactly advertise the icky parts of their business. 'Eating Animals' by Jonathan Safran Foer is a good place to start.
> According to the National Office of Animal Health (NOAH, 2001), antibiotic growth promoters are used to "help growing animals digest their food more efficiently, get maximum benefit from it and allow them to develop into strong and healthy individuals". Although the mechanism underpinning their action is unclear, it is believed that the antibiotics suppress sensitive populations of bacteria in the intestines. It has been estimated that as much as 6 per cent of the net energy in the pig diet could be lost due to microbial fermentation in the intestine (Jensen, 1998). If the microbial population could be better controlled, it is possible that the lost energy could be diverted to growth.
Do you have any material from a peer reviewed source that refutes this?
I'm not sure this is entirely true.