There are plenty of nutrients in milk that more than make up for an inability to digest lactose. Goats milk has less lactose than cows milk, easier to digest proteins and fats.
I would never waste a free protein source for my animals. We give the dogs, the cat, the chickens and sometimes even a rabbit or two extra milk.
My chickies (and other animals) are healthy, kept in clean conditions, and fed properly formulated rations for their purpose. Feeding milk as a treat isn't going to hurt them unless they are already susceptable to a bacterial overgrowth.
I don't disagree that it *may* be possible to serve as a medium for bacteria, but I do disagree that feeding it as a treat to healthy birds will cause any ill effects.