This because the disease had already become resistant to a previously used, different human antibiotic, streptomycin.
The Infectious Diseases Society of America tries to get them to see reason:
"At a time when bacteria are becoming increasingly resistant to many of our best antibiotics, it is an extremely bad idea to risk undermining gentamicin's effectiveness for treating human disease by using it to treat a disease in apples." (IDSA President Donald Poretz, MD in a press release.)Gentamicin is used against staph and against a range of Gram-negative bacteria, and is an important drug for bloodstream infections in newborns. In a bizarre irony, the EPA bans its use on imported fruits/vegetables — because of fears of fostering resistance.
The decision in the Federal Register here. The original EPA proposal here. A Clinical Infectious Diseases article about human antibiotic use in plant agriculture here. And somewhere in the immediate vicinity, me clutching my head and wandering away muttering.