I'm really surprised this isn't the standard everywhere. In NZ it's quite similar and very strict; if any trace of antibiotic is detected in a batch of milk it

I'm really surprised this isn't the standard everywhere. In NZ it's quite similar and very strict; if any trace of antibiotic is detected in a batch of milk it is rejected, and the offending farmer must pay for it (easily $1,000s, probably much more). Aside from the resistance issue, one reason for this is quite simple; antibiotics in the milk mean you can't make cheese with it. If a cow needs antibiotics, they are separated from the herd for a few days until they are clear (and also can't be slaughtered for meat for a longer period of time).