> The hives being robbed out are the seriously infested "nuisance beekeeping" hives, which is why they are so weak, they can be robbed out so easily. 

This is a logical assumption, but totally unprovable. You can't show anybody these  "nuisance" hives. On the other hand, recent work points in a different direction, to the practice of crowding hives in apiaries

This study shows that the crowding of honeybee
colonies in an apiary can boost the drifting of
drones between colonies. It also suggests that this
crowding can lead to healthy colonies suddenly
acquiring lethal infestations of Varroa mites when
other colonies in the same apiary are dying from
high levels of Varroa mites and viruses.

By showing
that differences in colony spacing, even on a
small scale (ca. 1 m vs. ca. 30 m), can strongly
influence the mite infestations of colonies, this
study complements a previous study that showed
that differences in colony spacing on a landscape
scale (in regions with low vs. high colony densities)
can strongly influence the mite infestations of
colonies (Frey and Rosenkranz 2014).

In the present study, when colonies were
housed in ways that are typical for apiculture—in
hives painted the same color, arranged in a row
and facing the same direction, and spaced
tightly—it was found in both years that
approximately 35 % of the drones entering each
colony’s hive did not match the color morph produced
by the colony.

Whatever the precise mechanism, it now seems
clear that when honeybee colonies are forced to
live in identical hives that are clustered in an
apiary, a high level of drifting can occur and
colonies can be vulnerable to V. destructor and
the viruses that the mites carry, especially if one or
more of the clustered colonies collapses.

Seeley, Thomas D., and Michael L. Smith. "Crowding honeybee colonies in apiaries can increase their vulnerability to the deadly ectoparasite Varroa destructor." Apidologie: 1-12.

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