It's their protein source; honey is pretty much carbs-only, good in winter, when they just keep warm, but raising kids on that is not good. Besides, bees are wasps, and wasps are carnivores, they need protein and mostly eat pollen protein in summer if they can. Honeybees eat flesh too, including cannibalism, just less often than plant material - apparently, with organized labor veganism turned out to be more effective, at least for them. No morals behind it, pure business.