I think the difference is that the problem of antibiotics overuse in US agriculture is amenable to a relatively simple regulatory situation: making a law banning or limiting antibiotics use in livestock. Addressing the issue in China, on the other hand, would require overhaul of the entire medical system, as hospitals are dependent on antibiotics sales for a significant proportion of their revenue.

I went with a friend to a hospital in Shanghai once, and the doctor ran a blood test and found that my friend had a viral infection. He then attempted to sell her antibiotics (which anyone who's done even high school biology knows are completely ineffective against viruses, only against bacteria). I called him out on it, and he just scowled at me and muttered 'westerners do things differently'.