Looking into robbing by honey bees (I prefer the terms "looting" or "pillaging") -- I found this:

Why do ant colonies participate in many symbioses and are sometimes largely dependent upon them, whereas honeybee colonies have no known symbionts but many parasites? Honey bees do not display territoriality in their feeding areas. In addition, when floral resources are scarce, foragers will often steal stored honey from relatively weak or dying colonies (Gary, 1966), also increasing opportunities for guests to switch host colony. It seems, therefore, that guests can easily transfer from one colony of honeybees to another. 

In Ewald’s words: ‘‘Even if a pathogen reproduces so extensively that it causes its host to become gravely sick, its host-impairing instructions may still win over the less damaging instructions of less aggressive competitors. The more virulent pathogen would achieve this success if its increased replication led to a level of transmission into new hosts that exceeded the loss of transmission resulting from the host’s illness or death.’’ (Ewald, 1993, p. 56). 

An extremely virulent and lethal variant of a pathogen, killing a large proportion of its hosts, will often find it increasingly difficult to transmit to new hosts, since these will get scarce and more thinly distributed. Hence, natural selection will start favoring milder variants of this pathogen. But if for some reason hosts do not become more scarce, then this automatic natural selection of milder variants of the pathogen will not occur. 

Journal of Theoretical Biology 247 (2007) 365–370 

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