The WHO isn't just giving advice to mothers in America, they're giving advice to mothers in Bangladesh and other developing countries. Indeed, that's their primary audience. The short-term advantages of breastfeeding are extremely relevant to mothers in countries where the water may not be clean and where babies are constantly exposed to high levels of pathogens. Those considerations are far less relevant to babies born in the first world.
It's stupid to take WHO recommendations, or medical recommendations generally, at face value, because they generally are devoid of any cost-benefit analysis. Take this study of short-term benefits done in the U.K. http://pediatrics.aappublications.org/content/119/4/e837.ful.... It found a 53% reduction in diarrhea hospitalizations from exclusive breastfeeding, and a 27% reduction in lower respiratory tract infections. This is a very strong result and justifies the "breast is best" recommendation. Yet, when subjected to cost-benefit analysis, the result isn't so clear. The incidence of these conditions in the sample of infants was only 1-3% to begin with. In other words, exclusive breastfeeding reduces the chance of a diarrhea hospitalization from 1.1% to 0.5%, and for a lower respiratory tract infection from 3.2% to 2.3%. And remember, both conditions are easily treatable.
On the other side of the scale, exclusive breastfeeding guarantees that the mother will be saddled with the primary role of feeding the infant for six months to a year and cannot easily delegate the task to the father (even with pumping, the father remains wholly dependent on the mother). Does a statistically significant, but small, reduction in an unlikely outcomes really justify blanket recommendations to exclusively breastfeed? Do you apply that same sort of extreme caution to every parenting choice? Given that motor vehicle crashes are the leading cause of death in children at every age from 2-14, shouldn't the WHO recommend that everyone move out of the suburbs into cities where they don't have to drive?