Misuse of antibiotics in pork

Nicholas Kristof reported in today's New York Times that fully seventy percent of antibiotic use in this country is on healthy livestock.  While patients are suffering with conditions for which doctors are loath to over-prescribe antibiotics, the food industry is creating an entire nation of antibiotic-resistant livestock.  By some counts, as much as five percent of our pork supply may be contaminated with deadly Methicillin-resistant staphylococcus aureus (MRSA).

As Kristof notes, under the circumstances it is unconscionable that the U.S., alone among developed countries, allows agribusiness to overuse antibiotics--for non-therapeutic use--in animal feed.  If we don't give antibiotics to health humans as a preventative measure, why do we tolerate it in our food supply?