From the Jack Newman Article:
"The mother synthesizes antibodies when she ingests, inhales or otherwise comes in contact with a disease-causing agent. Because the mother makes antibodies only to pathogens in her environment, the baby receives the protection it most needs-against the infectious agents it is most likely to encounter in the first weeks of life."
The mother is exposed to the same pathogens in the infant's environment, or from the baby herself. Those pathogens are generally acquired and processed in the upper aerodigestive tract, and then the mom makes immune factors to be secreted in the milk. (enteromammary circulation, or pathway) There just isn't a mechanism for acquisition of pathogens via the infant's saliva interacting directly with the breast. That's why the mom and babe need to be in the same environment to make this work.