It's really about the vector. Varroa mites are parasites but just like ticks they carry pathogens. Varroa mites feed on the fat and hemolymph but they also inject viral particles into the developing bee. Deformed Wing Virus, DWV, is lethal to a mite infested colony. So...

The Nov. 2016 article in Bee Culture Magazine did a great job on this topic. Here's a synopsis.

"When the U.S. Food and Drug Administration (FDA)’s Veterinary Feed Directive (VFD) regulations take effect in January to combat overuse of antimicrobial drugs in food animal...