My reference to, 'good farmers have good fences' means it protects not only their stock but those of their neighbours from disease.
Honeybees obviously fly so fences are of no use in keeping disease at bay.
So quite honestly, if there was a beekeeper in my neck of the woods who didn't treat their bees when necessary, I would certainly not be happy. I'm not talking about varroa here, for we all need to treat for this regardless.
Keep in mind, not every field bee may make it home and 'will ask' to enter the nearest colony. Therein lies the problem for others.